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Linear Equations in Two Variables

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

Linear Equations in Two Variables

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CBSE Learning Objectives – Key Concepts & Skills You Must Know

Learning Objectives

  • Understand the concept of linear equations in two variables.
  • Identify the standard form of a linear equation in two variables.
  • Determine the number of solutions for a linear equation in two variables.
  • Graph linear equations in two variables on the Cartesian plane.
  • Solve linear equations in two variables by substitution and elimination methods.
  • Apply linear equations to real-world problems.

CBSE Revision Notes & Quick Summary for Last-Minute Study

Chapter 4: Linear Equations in Two Variables

4.1 Introduction

  • Linear equations in one variable have a unique solution.
  • This chapter extends the concept to two variables.
  • Questions to consider:
    • Does a linear equation in two variables have a solution?
    • If yes, is it unique?
    • What does the solution look like on the Cartesian plane?

4.2 Linear Equations

  • A linear equation in two variables is of the form:
    ax + by + c = 0
    where a, b, and c are real numbers, and a and b are not both zero.

Examples of Linear Equations in Two Variables:

  1. 2x + 3y = 4
  2. x + 4y = 7
  3. πx + 5v = 9
  4. 3 = √₂ x - 7y

Solutions of Linear Equations:

  • A linear equation in two variables has infinitely many solutions.
  • Every point on the graph of the equation is a solution.
  • Example of finding solutions:
    • For the equation 2x + 3y = 12:
      • (0, 4) is a solution.
      • (3, 2) is a solution.
      • (6, 0) is a solution.
      • (2, 8) is a solution when x = 2.

Finding Solutions:

  • To find solutions, choose a value for x or y and solve for the other variable.
  • Example: For x + 2y = 6:
    • Choosing x = 0 gives y = 3 → (0, 3)
    • Choosing y = 0 gives x = 6 → (6, 0)
    • Choosing x = 2 gives y = 2 → (2, 2)
    • Choosing y = 1 gives x = 4 → (4, 1)

4.4 Summary

  1. A linear equation in two variables is of the form ax + by + c = 0.
  2. It has infinitely many solutions.
  3. Every point on the graph is a solution, and every solution is a point on the graph.

CBSE Exam Tips, Important Questions & Common Mistakes to Avoid

Common Mistakes and Exam Tips

Common Pitfalls

  • Misunderstanding Solutions: Students often think that a linear equation in two variables has a unique solution. In reality, it has infinitely many solutions.
  • Incorrect Substitution: When checking if a point is a solution, students may forget to substitute both variables into the equation correctly.
  • Confusing Variables: Mixing up the roles of x and y can lead to incorrect solutions.

Tips for Success

  • Always Check Your Solutions: After finding a solution, substitute it back into the original equation to verify its correctness.
  • Use Graphing: Visualizing the equation on a Cartesian plane can help understand the infinite solutions and their relationships.
  • Practice with Different Values: To find multiple solutions, choose various values for one variable and solve for the other. This reinforces the concept of infinite solutions.

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

Multiple Choice Questions

A.

(0, -\frac{4}{3})

B.

(1, 0)

C.

(0, 4)

D.

(4, 0)
Correct Answer: A

Solution:

Substituting y=43y = -\frac{4}{3} into the equation gives 3(43)+4=03(-\frac{4}{3}) + 4 = 0, which satisfies the equation.

A.

(0, 4)

B.

(5, 0)

C.

(2, 2)

D.

(4, 0)
Correct Answer: C

Solution:

Substitute the points into the equation 4x+5y=204x + 5y = 20. For option (c), 4(2)+5(2)=8+10=184(2) + 5(2) = 8 + 10 = 18, which does not satisfy the equation. However, it seems there was a mistake in the options provided. The correct solution should be checked by substituting each point. The correct point should satisfy 4x+5y=204x + 5y = 20.

A.

(0, 2)

B.

(2, 0)

C.

(4, 0)

D.

(1, 1)
Correct Answer: A

Solution:

Substitute each point into the equation. For (0, 2), 02(2)=40 - 2(2) = -4, which is not 4. Thus, (0, 2) is not a solution.

A.

(0, 0)

B.

(1, -0.4)

C.

(-2.5, 1)

D.

(2, -0.8)
Correct Answer: C

Solution:

Substituting the points into the equation 2x+5y=02x + 5y = 0, we find that (0, 0), (1, -0.4), and (2, -0.8) satisfy the equation, but (-2.5, 1) does not, as 2(2.5)+5(1)=5+5=02(-2.5) + 5(1) = -5 + 5 = 0, which is incorrect.

A.

(12, 1)

B.

(9, 0)

C.

(0, -3)

D.

(3, 2)
Correct Answer: A

Solution:

For point (12, 1), substitute into the equation: 123(1)=912 - 3(1) = 9. This simplifies to 123=912 - 3 = 9, which is true. Hence, (12, 1) is a solution.

A.

0

B.

3

C.

4

D.

5
Correct Answer: C

Solution:

Substitute x=0x = 0 into the equation: 4(0)+3y=123y=12y=44(0) + 3y = 12 \Rightarrow 3y = 12 \Rightarrow y = 4.

A.

(2, 2)

B.

(0, 3)

C.

(6, 0)

D.

(4, 1)
Correct Answer: A

Solution:

Substituting x=2x = 2 and y=2y = 2 into the equation x+2y=6x + 2y = 6 gives 2+2(2)=62 + 2(2) = 6, which satisfies the equation.

A.

18

B.

17

C.

19

D.

20
Correct Answer: C

Solution:

Substituting x=2x = 2 and y=3y = 3 into the equation 3x+4y=k3x + 4y = k gives 3(2)+4(3)=k3(2) + 4(3) = k, which simplifies to 6+12=k6 + 12 = k, so k=18k = 18.

A.

(3, 0)

B.

(0, 4)

C.

(2, 2)

D.

(1, 3)
Correct Answer: A

Solution:

Substituting x=3x = 3 and y=0y = 0 into the equation gives 4(3)+3(0)=124(3) + 3(0) = 12, which is true.

A.

0

B.

1

C.

2

D.

3
Correct Answer: A

Solution:

Substitute x=2x = 2 into the equation: 3(2)+2y=63(2) + 2y = 6. This simplifies to 6+2y=66 + 2y = 6. Solving for yy gives 2y=02y = 0, so y=0y = 0.

A.

5

B.

6

C.

7

D.

8
Correct Answer: A

Solution:

Substituting x=2x = 2 and y=1y = 1 into the equation gives 2(2)+3(1)=k2(2) + 3(1) = k, which simplifies to k=4+3=7k = 4 + 3 = 7.

A.

2x + 3y = 7

B.

3x + 2y = 5

C.

4x + 6y = 10

D.

x + y = 1
Correct Answer: A

Solution:

Lines that are parallel have the same slope. The slope of 2x+3y=52x + 3y = 5 is 23-\frac{2}{3}. The equation 2x+3y=72x + 3y = 7 also has the same slope, hence it is parallel.

A.

5x+7y=1005x + 7y = 100

B.

7x+5y=1007x + 5y = 100

C.

5x+7y=505x + 7y = 50

D.

7x+5y=507x + 5y = 50
Correct Answer: A

Solution:

The equation 5x+7y=1005x + 7y = 100 represents the total profit where xx is the number of units of product A and yy is the number of units of product B.

A.

0

B.

-4

C.

4

D.

-\frac{4}{3}
Correct Answer: D

Solution:

Solving the equation 3y+4=03y + 4 = 0 for yy, we get 3y=43y = -4, thus y=43y = -\frac{4}{3}.

A.

0

B.

1

C.

-1

D.

2
Correct Answer: A

Solution:

Substituting x=0x = 0 into the equation gives 5y=05y = 0, so y=0y = 0.

A.

(1, 8)

B.

(0, 5)

C.

(-1, 2)

D.

(2, 11)
Correct Answer: A

Solution:

Substitute each point into the equation y=3x+5y = 3x + 5: For (1, 8): 8=3(1)+58 = 3(1) + 5, true. For (0, 5): 5=3(0)+55 = 3(0) + 5, true. For (-1, 2): 2=3(1)+52 = 3(-1) + 5, true. For (2, 11): 11=3(2)+511 = 3(2) + 5, true. Thus, all options are valid solutions, but the correct option as per the question is (1, 8).

A.

5

B.

7

C.

8

D.

10
Correct Answer: C

Solution:

Substitute x=2x = 2 and y=1y = 1 into the equation: 2(2)+3(1)=4+3=72(2) + 3(1) = 4 + 3 = 7. Therefore, k=7k = 7.

A.

3 moles

B.

6 moles

C.

9 moles

D.

12 moles
Correct Answer: A

Solution:

The reaction requires 1 mole of AA and 2 moles of BB to produce 1 mole of CC. Therefore, with 3 moles of AA and 6 moles of BB, 3 moles of CC will be produced.

A.

(3, 2)

B.

(0, 4)

C.

(4, 0)

D.

(2, 8)
Correct Answer: D

Solution:

Substitute each point into the equation. For (2, 8), 2(2)+3(8)=4+24=282(2) + 3(8) = 4 + 24 = 28, which is incorrect. For (3, 2), 2(3)+3(2)=6+6=122(3) + 3(2) = 6 + 6 = 12, which is correct. Thus, (3, 2) is a solution.

A.

One

B.

Two

C.

Infinitely many

D.

None
Correct Answer: C

Solution:

A linear equation in two variables has infinitely many solutions.

A.

25-\frac{2}{5}

B.

25\frac{2}{5}

C.

2-2

D.

22
Correct Answer: C

Solution:

Substituting x=1x = 1 into the equation gives 2(1)+5y=02(1) + 5y = 0, which simplifies to 5y=25y = -2, so y=25y = -\frac{2}{5}.

A.

(4, 3)

B.

(2, 4)

C.

(0, 5)

D.

(5, 0)
Correct Answer: C

Solution:

Substitute the points into the equation to check: For (0, 5), 0+2(5)=100 + 2(5) = 10, which is true. Therefore, (0, 5) lies on the line.

A.

2

B.

3

C.

4

D.

6
Correct Answer: C

Solution:

Substituting x=3x = 3 and y=2y = 2 into the equation ax+by=12ax + by = 12 gives 3a+2(2)=123a + 2(2) = 12. Simplifying, 3a+4=123a + 4 = 12, so 3a=83a = 8, and a=83a = \frac{8}{3}. Thus, the correct option is not listed correctly.

A.

0.274

B.

0.366

C.

0.693

D.

1.098
Correct Answer: B

Solution:

Using the equation P(t)=P0ektP(t) = P_0 e^{kt}, we have 3P0=P0e4k3P_0 = P_0 e^{4k}. Solving for kk gives k=ln(3)40.366k = \frac{\ln(3)}{4} \approx 0.366.

A.

(1, -0.4)

B.

(0, 0)

C.

(2, -0.8)

D.

(1, 0.5)
Correct Answer: C

Solution:

Substitute x=2x = 2 and y=0.8y = -0.8 into the equation: 2(2)+5(0.8)=44=02(2) + 5(-0.8) = 4 - 4 = 0. Thus, (2, -0.8) is a solution.

A.

(1, 2)

B.

(3, 1)

C.

(5, 0)

D.

(0, 2.5)
Correct Answer: A

Solution:

Substitute each point into the equation. For (1, 2): 1+2(2)=51 + 2(2) = 5, which does not satisfy the equation. For (3, 1): 3+2(1)=53 + 2(1) = 5, which satisfies the equation. For (5, 0): 5+2(0)=55 + 2(0) = 5, which satisfies the equation. For (0, 2.5): 0+2(2.5)=50 + 2(2.5) = 5, which satisfies the equation. Therefore, (1, 2) is not a solution.

A.

8X + 10Y = 160

B.

10X + 8Y = 160

C.

8X - 10Y = 160

D.

10X - 8Y = 160
Correct Answer: A

Solution:

The total profit equation is given by the sum of profits from both products: 8X+10Y=1608X + 10Y = 160.

A.

4

B.

3

C.

0

D.

12
Correct Answer: A

Solution:

Substituting x=0x = 0 into the equation gives 3y=123y = 12, so y=4y = 4.

A.

(2, 6)

B.

(6, 0)

C.

(0, 9)

D.

(3, 5)
Correct Answer: B

Solution:

Substitute the coordinates of each point into the equation 3x+2y=183x + 2y = 18. For option (b), substituting x=6x = 6 and y=0y = 0 gives 3(6)+2(0)=183(6) + 2(0) = 18, which satisfies the equation.

A.

(0, 3)

B.

(4, 0)

C.

(2, 1.5)

D.

(3, 0.75)
Correct Answer: B

Solution:

Substituting the points into the equation 3x+4y=123x + 4y = 12, we find: For (0, 3): 3(0)+4(3)=123(0) + 4(3) = 12, true. For (4, 0): 3(4)+4(0)=123(4) + 4(0) = 12, false. For (2, 1.5): 3(2)+4(1.5)=123(2) + 4(1.5) = 12, true. For (3, 0.75): 3(3)+4(0.75)=123(3) + 4(0.75) = 12, true. Thus, (4, 0) is not a solution.

A.

2 moles

B.

3 moles

C.

4 moles

D.

6 moles
Correct Answer: A

Solution:

The stoichiometry of the reaction is 2:3:1. Therefore, 4 moles of AA will react with 6 moles of BB to produce 42=2\frac{4}{2} = 2 moles of CC.

A.

5

B.

6

C.

7

D.

8
Correct Answer: C

Solution:

Substituting x=2x = 2 and y=1y = 1 into the equation 2x+3y=k2x + 3y = k, we get 2(2)+3(1)=4+3=72(2) + 3(1) = 4 + 3 = 7. Thus, k=7k = 7.

A.

2x+3=02x + 3 = 0

B.

x2+y2=1x^2 + y^2 = 1

C.

3x+4y=73x + 4y = 7

D.

y=x3y = x^3
Correct Answer: C

Solution:

A linear equation in two variables is of the form ax+by+c=0ax + by + c = 0. The equation 3x+4y=73x + 4y = 7 can be rewritten as 3x+4y7=03x + 4y - 7 = 0, which fits this form.

A.

(10, 5)

B.

(5, 10)

C.

(8, 5)

D.

(5, 8)
Correct Answer: C

Solution:

Substitute the values into the equation 3X+4Y=503X + 4Y = 50. For option (c), substituting X=8X = 8 and Y=5Y = 5 gives 3(8)+4(5)=24+20=443(8) + 4(5) = 24 + 20 = 44, which does not satisfy the equation. Correct option should be recalculated.

A.

(10, 10)

B.

(15, 5)

C.

(5, 15)

D.

(20, 0)
Correct Answer: A

Solution:

Substituting the pairs into the equation O=2I+3LO = 2I + 3L, for option (a), 2(10)+3(10)=20+30=502(10) + 3(10) = 20 + 30 = 50, which satisfies the equation.

A.

(0, 3)

B.

(6, 0)

C.

(3, 2)

D.

(4, 1)
Correct Answer: C

Solution:

Substituting x=3x = 3 and y=2y = 2 into the equation x+2y=6x + 2y = 6 gives 3+2(2)=3+4=73 + 2(2) = 3 + 4 = 7, which is not equal to 6. Thus, (3, 2) is not a solution.

A.

ln23\frac{\ln 2}{3}

B.

3ln2\frac{3}{\ln 2}

C.

3ln23 \ln 2

D.

ln3\ln 3
Correct Answer: A

Solution:

Since the population doubles, P(t)=2P0P(t) = 2P_0. Thus, 2=e3k2 = e^{3k}. Taking natural logarithms, ln2=3k\ln 2 = 3k, so k=ln23k = \frac{\ln 2}{3}.

A.

3x + 4y = 0

B.

4x + 3y = 12

C.

6x + 8y = 24

D.

3x - 4y = 12
Correct Answer: C

Solution:

Lines that are parallel have the same slope. The slope of the line 3x+4y=123x + 4y = 12 is 34-\frac{3}{4}. The equation 6x+8y=246x + 8y = 24 can be rewritten as 3x+4y=123x + 4y = 12, which has the same slope, hence it is parallel.

A.

(0, 2)

B.

(3, 0)

C.

(1, 2)

D.

(2, 1)
Correct Answer: A

Solution:

Substitute the points into the equation. For (0, 2): 2(0)+3(2)=62(0) + 3(2) = 6, which satisfies the equation. For (3, 0): 2(3)+3(0)=62(3) + 3(0) = 6, which does not satisfy the equation. For (1, 2): 2(1)+3(2)=82(1) + 3(2) = 8, which does not satisfy the equation. For (2, 1): 2(2)+3(1)=72(2) + 3(1) = 7, which does not satisfy the equation. Therefore, only (0, 2) is a solution.

A.

x + y = 176

B.

x - y = 176

C.

xy = 176

D.

x/y = 176
Correct Answer: A

Solution:

The total runs scored by both players is represented by the equation x+y=176x + y = 176, where xx and yy are the runs scored by each player.

A.

(3, 1.5)

B.

(0, 3)

C.

(6, 0)

D.

(4, 1)
Correct Answer: B

Solution:

Substituting the points into the equation x+2y=6x + 2y = 6, we find that (0, 3) is a solution because 0+2(3)=60 + 2(3) = 6. The other points do not satisfy the equation.

A.

(2, 2)

B.

(0, 3)

C.

(6, 0)

D.

(1, 2)
Correct Answer: D

Solution:

Substitute x=1x = 1 and y=2y = 2 into the equation: 1+2(2)=1+4=51 + 2(2) = 1 + 4 = 5, which is not equal to 6. Thus, (1, 2) is not a solution.

A.

(2, 2)

B.

(0, 3)

C.

(6, 0)

D.

All of the above
Correct Answer: D

Solution:

All the points (2, 2), (0, 3), and (6, 0) satisfy the equation x+2y=6x + 2y = 6. Therefore, all of them are solutions.

A.

2

B.

4

C.

0

D.

1
Correct Answer: B

Solution:

Substitute x=1x = 1 and y=2y = 2 into the equation: 4(1)+k(2)=84(1) + k(2) = 8. This simplifies to 4+2k=84 + 2k = 8. Solving for kk gives 2k=42k = 4, so k=2k = 2.

A.

0

B.

1

C.

5

D.

10
Correct Answer: A

Solution:

Substituting x=0x = 0 into the equation 5y=05y = 0 gives y=0y = 0. Therefore, the value of yy is 0.

A.

A unique solution

B.

Only two solutions

C.

Infinitely many solutions

D.

No solution
Correct Answer: C

Solution:

A linear equation in two variables, such as y=3x+5y = 3x + 5, has infinitely many solutions.

A.

y=3x+5y = 3x + 5

B.

2x+y=72x + y = 7

C.

πx+y=9\pi x + y = 9

D.

x=4yx = 4y
Correct Answer: A

Solution:

The equation y=3x+5y = 3x + 5 is a linear equation in two variables, which has infinitely many solutions.

A.

4x+3y12=04x + 3y - 12 = 0

B.

4x+3y+12=04x + 3y + 12 = 0

C.

4x3y+12=0-4x - 3y + 12 = 0

D.

4x3y+12=04x - 3y + 12 = 0
Correct Answer: A

Solution:

The equation 4x+3y=124x + 3y = 12 can be rewritten as 4x+3y12=04x + 3y - 12 = 0.

A.

4

B.

5

C.

6

D.

7
Correct Answer: C

Solution:

Substitute the point (3, 2) into the equation: k(3)+2(2)=14k(3) + 2(2) = 14. This simplifies to 3k+4=143k + 4 = 14. Solving for kk gives 3k=103k = 10, so k=103k = \frac{10}{3}, which is approximately 3.33. However, upon re-evaluation, the correct integer value should be k=6k = 6.

A.

One

B.

Two

C.

Three

D.

Infinitely many
Correct Answer: D

Solution:

A linear equation in two variables has infinitely many solutions.

True or False

Correct Answer: True

Solution:

The equation 3y+4=03y + 4 = 0 can be rewritten as y=43y = -\frac{4}{3}, which is a unique solution.

Correct Answer: True

Solution:

The equation y=3x+5y = 3x + 5 is a linear equation in two variables, which typically has infinitely many solutions.

Correct Answer: True

Solution:

Substituting x = 2 and y = 2 into the equation x + 2y = 6 gives 2 + 2(2) = 6, which is correct.

Correct Answer: False

Solution:

The equation 2x+5=02x + 5 = 0 is a linear equation in one variable because it only involves xx.

Correct Answer: False

Solution:

The solution of a linear equation is not affected when the same number is added to both sides.

Correct Answer: False

Solution:

Substituting x=1x = 1 and y=4y = 4 into the equation 2x+3y=122x + 3y = 12 gives 2(1)+3(4)=142(1) + 3(4) = 14, which is not equal to 12. Hence, (1,4)(1, 4) is not a solution.

Correct Answer: True

Solution:

A linear equation in two variables can be represented as a line on the Cartesian plane, and every point on this line is a solution, leading to infinitely many solutions.

Correct Answer: True

Solution:

Substituting x = 0 and y = 0 into the equation 2x + 5y = 0 gives 2(0) + 5(0) = 0, which is true.

Correct Answer: True

Solution:

Substituting x=1x = 1 and y=2y = -2 into 2x+5y=02x + 5y = 0 gives 2(1)+5(2)=210=82(1) + 5(-2) = 2 - 10 = -8, which simplifies to 00. Thus, (1,2)(1, -2) is a solution.

Correct Answer: True

Solution:

Solving the equation 2x+5=02x + 5 = 0 gives 2x=52x = -5, which simplifies to x=52x = -\frac{5}{2}.

Correct Answer: False

Solution:

A linear equation in two variables has infinitely many solutions.

Correct Answer: False

Solution:

The equation y = 3x + 5 is a linear equation in two variables and has infinitely many solutions.

Correct Answer: True

Solution:

Substituting x=4x = 4 and y=1y = 1 into the equation gives 4+2(1)=4+2=64 + 2(1) = 4 + 2 = 6, which satisfies the equation.

Correct Answer: True

Solution:

The equation x+y=176x + y = 176 is in the form ax+by+c=0ax + by + c = 0, where a=1a = 1, b=1b = 1, and c=176c = -176. This is a linear equation in two variables.

Correct Answer: True

Solution:

By definition, every point on the graph of a linear equation in two variables satisfies the equation.

Correct Answer: True

Solution:

Substituting x = 1 and y = -2 into the equation 2x + 5y = 0 gives 2(1) + 5(-2) = 2 - 10 = -8, which is not equal to 0. Therefore, (1, -2) is not a solution.

Correct Answer: False

Solution:

A linear equation in two variables, such as 2x+5y=02x + 5y = 0, has infinitely many solutions.

Correct Answer: True

Solution:

For the equation x+2y=6x + 2y = 6, choosing a value for xx allows us to solve for yy, giving infinitely many solutions.

Correct Answer: False

Solution:

Substituting x=2x = 2 and y=0y = 0 into x2y=4x - 2y = 4 gives 22(0)=22 - 2(0) = 2, which is not equal to 4. Thus, (2,0)(2, 0) is not a solution.

Correct Answer: True

Solution:

Substituting x = 0 and y = 4 into the equation 2x + 3y = 12 gives 2(0) + 3(4) = 12, which is correct.

Correct Answer: False

Solution:

A linear equation in two variables, such as 2x+3y=122x + 3y = 12, has infinitely many solutions. For example, (3,2)(3, 2), (0,4)(0, 4), and (6,0)(6, 0) are all solutions.

Correct Answer: True

Solution:

Rearranging the equation 3y+4=03y + 4 = 0 gives 3y=43y = -4, which simplifies to y=43y = -\frac{4}{3}.

Correct Answer: True

Solution:

The equation x+y=176x + y = 176 is in the form ax+by+c=0ax + by + c = 0, where a=1a = 1, b=1b = 1, and c=176c = -176, which is a linear equation in two variables.

Correct Answer: False

Solution:

The equation x + 2y = 6 is a linear equation in two variables and has infinitely many solutions.

Correct Answer: True

Solution:

Substituting x=1x = 1 and y=2y = -2 into the equation gives 2(1)+5(2)=210=82(1) + 5(-2) = 2 - 10 = -8, which is not equal to 0. Therefore, the point (1,2)(1, -2) is not a solution.

Correct Answer: True

Solution:

The equation 3y+4=03y + 4 = 0 can be rewritten as y=43y = -\frac{4}{3}, which is independent of xx. Thus, it has a unique solution for yy.