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Pair of Linear Equations in Two Variables

CBSE notes, revision, important questions, MCQs, mock tests & result analytics

Pair of Linear Equations in Two Variables

Solving Linear Equations
Graphical Representation of Linear Equations
Consistency and Solutions of Linear Equations
Formulating Linear Equations from Word Problems
Algebraic Methods for Solving Equations
Interpreting Solutions of Linear Equations
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CBSE Learning Objectives – Key Concepts & Skills You Must Know

Learning Objectives

  • Understand how to solve pairs of linear equations using substitution, elimination, and graphical methods.
  • Learn to represent linear equations graphically and interpret the intersection, parallelism, or coincidence of lines.
  • Determine the consistency of linear equations and the nature of their solutions (unique, none, or infinite).
  • Translate real-world scenarios into linear equations and solve them to find unknown values.
  • Use algebraic techniques such as substitution and elimination to solve linear equations.
  • Analyze the solutions of linear equations to understand their implications in real-world contexts.

CBSE Revision Notes & Quick Summary for Last-Minute Study

Chapter Notes

Solving Linear Equations

  • Linear Equations in Two Variables: Equations of the form ax+by+c=0ax + by + c = 0.
  • Methods:
    • Substitution Method: Solve one equation for one variable and substitute into the other.
    • Elimination Method: Multiply equations to align coefficients and eliminate one variable by addition or subtraction.
    • Graphical Method: Plot equations on a graph to find intersections.

Graphical Representation of Linear Equations

  • Intersection: Lines intersect at a point, indicating a unique solution.
  • Parallel Lines: No intersection, indicating no solution.
  • Coincident Lines: Lines overlap, indicating infinitely many solutions.

Consistency and Solutions of Linear Equations

  • Consistent Equations: Have at least one solution.
  • Inconsistent Equations: Have no solution.
  • Dependent Equations: Have infinitely many solutions.

Formulating Linear Equations from Word Problems

  • Translate scenarios into mathematical equations.
  • Example: "Five pencils and seven pens cost ₹50, while seven pencils and five pens cost ₹46."

Algebraic Methods for Solving Equations

  • Substitution: Solve for one variable and substitute.
  • Elimination: Align coefficients, add/subtract to eliminate a variable.

Interpreting Solutions of Linear Equations

  • Unique Solution: Indicates a specific solution point.
  • No Solution: Parallel lines, no intersection.
  • Infinite Solutions: Coincident lines, overlap completely.

Examples

  • Example 1: Solve x+y=5x + y = 5 and 2x3y=42x - 3y = 4 using elimination.
  • Example 2: Graphically solve x+3y=6x + 3y = 6 and 2x3y=122x - 3y = 12.
  • Example 3: Formulate and solve equations from word problems, e.g., age-related problems or cost calculations.

Verification

  • Always verify solutions by substituting back into the original equations to ensure correctness.

CBSE Exam Tips, Important Questions & Common Mistakes to Avoid

Common Mistakes & Exam Tips

Solving Linear Equations

  • Mistake: Students often make errors in arithmetic calculations when using substitution or elimination methods.
    • Tip: Double-check each step of your calculations, especially when dealing with fractions or decimals.
  • Mistake: Incorrectly isolating variables in the substitution method.
    • Tip: Always ensure that one variable is completely isolated before substituting it into the other equation.

Graphical Representation of Linear Equations

  • Mistake: Misreading the graph when the solution involves non-integral coordinates.
    • Tip: Use algebraic methods to verify graphical solutions, especially when coordinates are not whole numbers.
  • Mistake: Failing to plot enough points to accurately draw the lines.
    • Tip: Plot at least two points for each line and ensure they are correctly calculated to draw accurate graphs.

Consistency and Solutions of Linear Equations

  • Mistake: Confusing the conditions for consistency, inconsistency, and dependency.
    • Tip: Remember:
      • Intersecting lines have a unique solution (consistent).
      • Parallel lines have no solution (inconsistent).
      • Coincident lines have infinitely many solutions (dependent and consistent).

Formulating Linear Equations from Word Problems

  • Mistake: Misinterpreting the problem statement leading to incorrect equations.
    • Tip: Carefully translate each part of the word problem into mathematical expressions, and verify the logic before solving.

Algebraic Methods for Solving Equations

  • Mistake: Skipping steps in the elimination method, leading to errors.
    • Tip: Follow each step methodically: equalize coefficients, eliminate one variable, solve for the other, and back-substitute.

Interpreting Solutions of Linear Equations

  • Mistake: Ignoring the context of the problem when interpreting solutions.
    • Tip: Always relate the solution back to the original problem to ensure it makes sense in the given context.
By being aware of these common pitfalls and following the provided tips, students can improve their accuracy and efficiency in solving linear equations.

CBSE Quiz & Practice Test – MCQs, True/False Questions with Solutions

Multiple Choice Questions

A.

1

B.

2

C.

3

D.

4
Correct Answer: A

Solution:

Let the number of pants be xx and the number of skirts be yy. We have the equations: y=2x2y = 2x - 2 and y=4x4y = 4x - 4. Solving these, we find x=1x = 1 and y=0y = 0. Thus, Champa bought 1 pant.

Chapter Concept:

Formulating Linear Equations from Word Problems

A.

Substitution Method

B.

Elimination Method

C.

Graphical Method

D.

Matrix Method
Correct Answer: B

Solution:

The elimination method involves adding or subtracting equations to eliminate one variable, making it easier to solve for the other.

Chapter Concept:

Algebraic Methods for Solving Equations

A.

Unique solution

B.

No solution

C.

Infinitely many solutions

D.

Cannot be determined
Correct Answer: C

Solution:

The second equation is a multiple of the first, indicating that the lines are coincident. Therefore, there are infinitely many solutions.

Chapter Concept:

Consistency and Solutions of Linear Equations

A.

They have no solution.

B.

They have a unique solution.

C.

They have infinitely many solutions.

D.

They form a triangle.
Correct Answer: C

Solution:

Dependent linear equations coincide, meaning they have infinitely many solutions.

Chapter Concept:

Graphical Representation of Linear Equations

A.

They intersect at a single point.

B.

They are parallel.

C.

They coincide.

D.

They form a right angle.
Correct Answer: C

Solution:

The second equation is a multiple of the first, indicating that the lines coincide, representing infinitely many solutions.

Chapter Concept:

Graphical Representation of Linear Equations

A.

They intersect at a single point.

B.

They are parallel.

C.

They coincide.

D.

They do not intersect.
Correct Answer: A

Solution:

The lines intersect at a single point, indicating a unique solution.

Chapter Concept:

Graphical Representation of Linear Equations

A.

Unique solution

B.

No solution

C.

Infinitely many solutions

D.

Dependent equations
Correct Answer: C

Solution:

The equations are coincident after manipulation, indicating infinitely many solutions.

Chapter Concept:

Interpreting Solutions of Linear Equations

A.

35\frac{3}{5}

B.

45\frac{4}{5}

C.

57\frac{5}{7}

D.

79\frac{7}{9}
Correct Answer: A

Solution:

Let the fraction be xy\frac{x}{y}. The equations are: x+2y+2=911\frac{x+2}{y+2} = \frac{9}{11} and x+3y+3=56\frac{x+3}{y+3} = \frac{5}{6}. Solving these, we find the fraction is 35\frac{3}{5}.

Chapter Concept:

Formulating Linear Equations from Word Problems

A.

Unique solution

B.

Infinitely many solutions

C.

No solution

D.

The equations are inconsistent
Correct Answer: C

Solution:

The equations are parallel lines, as shown by the ratios of coefficients, indicating no solution.

Chapter Concept:

Solving Linear Equations

A.

Substitution Method

B.

Elimination Method

C.

Both A and B

D.

None of the above
Correct Answer: C

Solution:

Both substitution and elimination methods are algebraic methods that can be used to solve linear equations when the graphical method is not convenient.

Chapter Concept:

Consistency and Solutions of Linear Equations

A.

The equations have a unique solution.

B.

The equations have no solution.

C.

The equations have infinitely many solutions.

D.

The equations are dependent.
Correct Answer: B

Solution:

Parallel lines indicate that the equations are inconsistent and have no solution.

Chapter Concept:

Solving Linear Equations

A.

They intersect at one point.

B.

They are parallel.

C.

They are coincident.

D.

They form a right angle.
Correct Answer: C

Solution:

The equations 3x+2y=53x + 2y = 5 and 6x+4y=106x + 4y = 10 are multiples of each other, indicating that the lines are coincident and have infinitely many solutions.

Chapter Concept:

Algebraic Methods for Solving Equations

A.

10

B.

15

C.

20

D.

5
Correct Answer: B

Solution:

Let the number of ₹ 50 notes be xx and the number of ₹ 100 notes be yy. We have the equations: x+y=25x + y = 25 and 50x+100y=200050x + 100y = 2000. Solving these, we find x=15x = 15 and y=10y = 10. Therefore, Meena received 15 ₹ 50 notes.

Chapter Concept:

Solving Linear Equations

A.

They have a unique solution.

B.

They have no solution.

C.

They have infinitely many solutions.

D.

They are inconsistent.
Correct Answer: A

Solution:

The lines intersect at a single point, indicating a unique solution, making the pair of equations consistent.

Chapter Concept:

Interpreting Solutions of Linear Equations

A.

Graphical method; solution is x=2x = 2, y=3y = 3

B.

Substitution method; solution is x=4x = 4, y=2y = 2

C.

Elimination method; solution is x=4x = 4, y=2y = 2

D.

None of the above
Correct Answer: C

Solution:

The elimination method is efficient because it allows us to eliminate one variable by adding or subtracting the equations. Solving, we find x=4x = 4 and y=2y = 2.

Chapter Concept:

Solving Linear Equations

A.

Unique solution

B.

No solution

C.

Infinitely many solutions

D.

Exactly two solutions
Correct Answer: C

Solution:

When the lines are coincident, there are infinitely many solutions as each point on the line is a solution.

Chapter Concept:

Consistency and Solutions of Linear Equations

A.

Intersecting lines

B.

Parallel lines

C.

Coincident lines

D.

None of the above
Correct Answer: C

Solution:

Both equations are equivalent, meaning the lines are coincident and have infinitely many solutions.

Chapter Concept:

Consistency and Solutions of Linear Equations

A.

The lines will intersect at a point.

B.

The lines will be parallel.

C.

The lines will coincide.

D.

The lines will form a triangle with the x-axis.
Correct Answer: C

Solution:

The second equation is a multiple of the first, indicating that the lines will coincide.

Chapter Concept:

Consistency and Solutions of Linear Equations

A.

₹ 9

B.

₹ 12

C.

₹ 15

D.

₹ 18
Correct Answer: A

Solution:

Let the fixed charge be ff and the additional charge per day be cc. We have the equations: f+4c=27f + 4c = 27 and f+2c=21f + 2c = 21. Solving these, we find f=9f = 9 and c=6c = 6. Thus, the fixed charge is ₹ 9.

Chapter Concept:

Formulating Linear Equations from Word Problems

A.

Unique solution

B.

No solution

C.

Infinitely many solutions

D.

Dependent solution
Correct Answer: A

Solution:

The lines intersect at a single point, providing a unique solution.

Chapter Concept:

Graphical Representation of Linear Equations

A.

Each pencil costs ₹2 and each eraser costs ₹1

B.

Each pencil costs ₹1 and each eraser costs ₹2

C.

The equations are dependent, and there are infinitely many solutions

D.

The equations are inconsistent, and there is no solution
Correct Answer: C

Solution:

Both equations represent the same line, indicating they are dependent and consistent with infinitely many solutions.

Chapter Concept:

Solving Linear Equations

A.

Multiply the second equation by 5

B.

Multiply the second equation by 3

C.

Add the two equations

D.

Subtract the first equation from the second
Correct Answer: B

Solution:

Multiplying the second equation by 3 makes it identical to the first, indicating coincident lines with infinitely many solutions.

Chapter Concept:

Solving Linear Equations

A.

2x+3y=92x + 3y = 9 and 4x+6y=184x + 6y = 18

B.

x+2y=4x + 2y = 4 and 2x+4y=122x + 4y = 12

C.

5x8y+1=05x - 8y + 1 = 0 and 3x245y+35=03x - \frac{24}{5}y + \frac{3}{5} = 0

D.

x+3y=6x + 3y = 6 and 2x3y=122x - 3y = 12
Correct Answer: A

Solution:

The equations 2x+3y=92x + 3y = 9 and 4x+6y=184x + 6y = 18 are multiples of each other, indicating coincident lines with infinitely many solutions.

Chapter Concept:

Graphical Representation of Linear Equations

A.

24

B.

42

C.

35

D.

53
Correct Answer: A

Solution:

Let the digits be xx and yy. The equations are: 10x+y+10y+x=6610x + y + 10y + x = 66 and xy=2x - y = 2. Solving these, we find the number is 24.

Chapter Concept:

Formulating Linear Equations from Word Problems

A.

The system has a unique solution.

B.

The system has infinitely many solutions.

C.

The system has no solution.

D.

The system is inconsistent.
Correct Answer: B

Solution:

The equations are multiples of each other, indicating they are coincident lines and have infinitely many solutions.

Chapter Concept:

Algebraic Methods for Solving Equations

A.

x=4,y=3x = 4, y = 3

B.

x=2,y=3x = 2, y = 3

C.

x=3,y=2x = 3, y = 2

D.

x=5,y=2x = 5, y = 2
Correct Answer: A

Solution:

Substitute x=2yx = 2y from the second equation into the first to get 3(2y)+4y=203(2y) + 4y = 20, solving gives y=3y = 3 and x=6x = 6.

Chapter Concept:

Solving Linear Equations

A.

Fixed charge = ₹30, Additional charge per hour = ₹5

B.

Fixed charge = ₹20, Additional charge per hour = ₹10

C.

Fixed charge = ₹40, Additional charge per hour = ₹2

D.

Fixed charge = ₹25, Additional charge per hour = ₹5
Correct Answer: A

Solution:

Let the fixed charge be ₹x and the additional charge per hour be ₹y. From the given conditions, we have the equations: x+3y=50x + 3y = 50 and x+7y=70x + 7y = 70. Solving these, we subtract the first equation from the second to get 4y=204y = 20, so y=5y = 5. Substituting back, we find x=30x = 30. Thus, the fixed charge is ₹30 and the additional charge per hour is ₹5.

Chapter Concept:

Solving Linear Equations

A.

Intersecting lines

B.

Parallel lines

C.

Coincident lines

D.

None of the above
Correct Answer: C

Solution:

The lines are coincident because the second equation is a multiple of the first, indicating infinitely many solutions.

Chapter Concept:

Solving Linear Equations

A.

They intersect at a single point.

B.

They are parallel.

C.

They coincide.

D.

They do not intersect.
Correct Answer: C

Solution:

The second equation is a multiple of the first, indicating that the lines coincide, representing infinitely many solutions.

Chapter Concept:

Graphical Representation of Linear Equations

A.

The lines intersect at a single point.

B.

The lines are parallel.

C.

The lines coincide.

D.

The lines form a triangle with the x-axis.
Correct Answer: C

Solution:

The equations are multiples of each other, indicating that the lines coincide, representing infinitely many solutions.

Chapter Concept:

Graphical Representation of Linear Equations

A.

2 rides and 4 games

B.

4 rides and 2 games

C.

3 rides and 3 games

D.

5 rides and 1 game
Correct Answer: B

Solution:

Let the number of rides be xx and games be yy. We have x=2yx = 2y and 3x+4y=203x + 4y = 20. Solving these, we find x=4x = 4 and y=2y = 2.

Chapter Concept:

Solving Linear Equations

A.

2x+3y=92x + 3y = 9 and 4x+6y=184x + 6y = 18

B.

x+2y=4x + 2y = 4 and 2x+4y=82x + 4y = 8

C.

3x+4y=203x + 4y = 20 and x2y=0x - 2y = 0

D.

x+y=5x + y = 5 and 2x+2y=102x + 2y = 10
Correct Answer: C

Solution:

The pair of equations 3x+4y=203x + 4y = 20 and x2y=0x - 2y = 0 are consistent and intersect at a point, hence they have a unique solution. The other pairs are either coincident or parallel.

Chapter Concept:

Algebraic Methods for Solving Equations

A.

10

B.

15

C.

5

D.

20
Correct Answer: A

Solution:

Let the number of ₹ 50 notes be xx and the number of ₹ 100 notes be yy. We have the equations: x+y=25x + y = 25 and 50x+100y=200050x + 100y = 2000. Solving these, we find x=10x = 10 and y=15y = 15. Thus, Meena received 10 notes of ₹ 50.

Chapter Concept:

Formulating Linear Equations from Word Problems

A.

Substitution method

B.

Elimination method

C.

Graphical method

D.

None of the above
Correct Answer: D

Solution:

The equations have no solution as they are inconsistent. The elimination method shows this by resulting in a false statement.

Chapter Concept:

Solving Linear Equations

A.

₹ 5

B.

₹ 6

C.

₹ 7

D.

₹ 8
Correct Answer: B

Solution:

Let the fixed charge be ff and the charge per km be cc. We have the equations: f+10c=105f + 10c = 105 and f+15c=155f + 15c = 155. Solving these, we find f=55f = 55 and c=6c = 6. Thus, the charge per km is ₹ 6.

Chapter Concept:

Formulating Linear Equations from Word Problems

A.

Find the value of one variable in terms of the other.

B.

Add the equations together.

C.

Graph the equations.

D.

Multiply the equations by constants.
Correct Answer: A

Solution:

The first step in the substitution method is to express one variable in terms of the other using one of the equations.

Chapter Concept:

Algebraic Methods for Solving Equations

A.

The equations are consistent with a unique solution.

B.

The equations are inconsistent with no solution.

C.

The equations are dependent with infinitely many solutions.

D.

The equations are consistent with multiple solutions.
Correct Answer: A

Solution:

The lines intersect at one point, indicating a unique solution, hence the equations are consistent.

Chapter Concept:

Solving Linear Equations

A.

The system has a unique solution.

B.

The system has infinitely many solutions.

C.

The system has no solution.

D.

The system is dependent.
Correct Answer: C

Solution:

Parallel lines indicate that the system of equations is inconsistent and has no solution.

Chapter Concept:

Algebraic Methods for Solving Equations

A.

Intersecting lines

B.

Parallel lines

C.

Coincident lines

D.

None of the above
Correct Answer: B

Solution:

The lines are parallel, as they have the same slope but different intercepts, indicating no solution.

Chapter Concept:

Interpreting Solutions of Linear Equations

A.

The pair of equations has a unique solution.

B.

The pair of equations has infinitely many solutions.

C.

The pair of equations has no solution.

D.

The pair of equations is inconsistent.
Correct Answer: B

Solution:

The equations 3x+4y=203x + 4y = 20 and 6x+8y=406x + 8y = 40 are multiples of each other, indicating they represent the same line. Therefore, they have infinitely many solutions, making them dependent and consistent.

Chapter Concept:

Solving Linear Equations

A.

Unique solution

B.

No solution

C.

Infinitely many solutions

D.

Cannot be determined
Correct Answer: A

Solution:

The lines intersect at a single point, indicating a unique solution.

Chapter Concept:

Consistency and Solutions of Linear Equations

A.

45\frac{4}{5}

B.

56\frac{5}{6}

C.

79\frac{7}{9}

D.

34\frac{3}{4}
Correct Answer: A

Solution:

Let the fraction be xy\frac{x}{y}. We have the equations: x+2y+2=911\frac{x+2}{y+2} = \frac{9}{11} and x+3y+3=56\frac{x+3}{y+3} = \frac{5}{6}. Solving these, we find x=4x = 4 and y=5y = 5. Thus, the original fraction is 45\frac{4}{5}.

Chapter Concept:

Formulating Linear Equations from Word Problems

A.

(3,1)(3, 1)

B.

(6,0)(6, 0)

C.

(0,2)(0, 2)

D.

(1,0)(1, 0)
Correct Answer: B

Solution:

Solving the equations x+3y=6x + 3y = 6 and 2x3y=122x - 3y = 12 simultaneously, we find the intersection point is (6,0)(6, 0). Thus, the solution is (6,0)(6, 0).

Chapter Concept:

Interpreting Solutions of Linear Equations

A.

3x+4y=203x + 4y = 20 and 6x+8y=406x + 8y = 40

B.

x+2y=4x + 2y = 4 and 2x+4y=82x + 4y = 8

C.

5x8y+1=05x - 8y + 1 = 0 and 15x24y+3=015x - 24y + 3 = 0

D.

2x+3y=92x + 3y = 9 and 4x+6y=184x + 6y = 18
Correct Answer: C

Solution:

The pair 5x8y+1=05x - 8y + 1 = 0 and 15x24y+3=015x - 24y + 3 = 0 are inconsistent because they represent parallel lines (same slope but different intercepts) and hence have no solution.

Chapter Concept:

Interpreting Solutions of Linear Equations

A.

They have a unique solution.

B.

They have no solution.

C.

They have infinitely many solutions.

D.

They are inconsistent.
Correct Answer: C

Solution:

Both equations are equivalent, hence they have infinitely many solutions.

Chapter Concept:

Formulating Linear Equations from Word Problems

A.

Unique solution

B.

No solution

C.

Infinitely many solutions

D.

Cannot be determined
Correct Answer: B

Solution:

The lines represented by the equations are parallel, hence the pair of equations has no solution.

Chapter Concept:

Consistency and Solutions of Linear Equations

A.

Consistent with a unique solution

B.

Inconsistent

C.

Consistent with infinitely many solutions

D.

Dependent and inconsistent
Correct Answer: C

Solution:

Both equations represent the same line, hence the system is consistent with infinitely many solutions.

Chapter Concept:

Consistency and Solutions of Linear Equations

A.

They are consistent with a unique solution.

B.

They are inconsistent.

C.

They are dependent and consistent.

D.

They have infinitely many solutions.
Correct Answer: B

Solution:

The equations are inconsistent because they lead to a false statement when one is subtracted from the other.

Chapter Concept:

Interpreting Solutions of Linear Equations

A.

₹ 9

B.

₹ 12

C.

₹ 15

D.

₹ 6
Correct Answer: A

Solution:

Let the fixed charge be xx and the additional charge per day be yy. We have the equations: x+4y=27x + 4y = 27 and x+2y=21x + 2y = 21. Solving these, we find x=9x = 9 and y=6y = 6. Therefore, the fixed charge is ₹ 9.

Chapter Concept:

Solving Linear Equations

A.

The lines intersect at a point.

B.

The lines coincide.

C.

The lines are parallel.

D.

The lines form a triangle.
Correct Answer: C

Solution:

If the lines are parallel, the pair of linear equations has no solution and is inconsistent.

Chapter Concept:

Graphical Representation of Linear Equations