**Chapter 2 Matrices Ex 2.3**

## Chapter 2 Matrices Ex 2.3

**Question 1.Solve the following equations by inversion method.(i) x + 2y = 2, 2x + 3y = 3Solution:**

The given equations can be written in the matrix form as :

This is of the form AX = B, where

∴ A

^{-1}= [−322−1]

Now, premultiply AX = B by A

^{-1}, we get,

A

^{-1}(AX) = A

^{-1}B

∴ (A

^{-1}A)X = A

^{-1}B

∴ IX = A

^{-1}B

By equality of matrices,

x = 0, y = 1 is the required solution.

**(ii) x + y = 4, 2x – y = 5Solution:**
x + y = 4, 2x – y = 5

The given equations can be written in the matrix form as:

|A| = -1 – 2 = -3 ≠ 0

By equality of matrices.

x = 3, y = 1

**(iii) 2x + 6y = 8, x + 3y = 5Solution:**
The given equations can be written in the matrix form as :

∴ A

^{-1}does not exist.

Hence, x and y do not exist.

**Question 2.Solve the following equations by reduction method.(i) 2x + y = 5, 3x + 5y = -3Solution:**

The given equations can be written in the matrix form as :

By equality of matrices,

2x + y = 5 …(1)

7y = -21 …(2)

From (2), y = -3

Substituting y = -3 in (1), we get,

2x – 3 = 5

∴ 2x = 8 ∴ x = 4

Hence, x = 4, y = -3 is the required solution.

**(ii) x + 3y = 2, 3x + 5y = 4.Solution:**

The given equations can be written in the matrix form as :

**(iii) 3x – y = 1, 4x + y = 6Solution:**
The given equations can be written in the matrix form as :

By equality of matrices,

12x – 4y = 4 … (1)

7y = 14 … (2)

From (2), y = 2

Substituting y = 2 in (1), we get,

12x – 8 = 4

∴ 12x = 12 ∴ x = 1

Hence, x = 1, y = 2 is the required solution.

**(iv) 5x + 2y = 4, 7x + 3y = 5Solution:**

5x + 2y = 4 ………..(1)

7x + 3y = 5 …………(2)

Multiplying Eq. (1) with 7 and Eq. (2) with 5

Put y = -3 into Eq. (1)

5x + 2y = 4

5x + 2(-3) = 4

5x – 6 = 4

5x = 4 + 6

5x = 10

x = 105

x = 2

Hence, x = 2, y = -3 is the required solution.

**Question 3.The cost of 4 pencils, 3 pens and 2 erasers is ₹ 60. The cost of 2 pencils, 4 pens and 6 erasers is ₹ 90, whereas the cost of 6 pencils, 2 pens and 3 erasers is ₹ 70. Find the cost of each item by using matrices.Solution:**

Let the cost of 1 pencil, 1 pen and 1 eraser be ₹ x, ₹ y and ₹ z respectively.

Then, from the given conditions,

4x + 3y + 2z = 60

2x + 4y + 6z = 90, i.e., x + 2y + 3z = 45

6x + 2y + 3z = 70

These equations can be written in the matrix form as :

By equality of matrices,

x + 2y + 3z = 45 …….(1)

– 5y – 10z = – 120 …….(2)

5z = 40

From (3), z = 8

Substituting z = 8 in (2), we get,

– 5y – 80 = -120

∴ – 5y = -40 ∴ y = 8

Substituting y = 8, z = 8 in (1), we get,

x + 16 + 24 = 45

∴ x + 40 = 45 ∴ x = 5

∴ x = 5, y = 8, z = 8

Hence, the cost is ₹ 5 for a pencil, ₹ 8 for a pen and ₹ 8 for an eraser.

**Question 4.If three numbers are added, their sum is 2. If 2 times the second number is subtracted from the sum of first and third numbers, we get 8 and if three times the first number is added to the sum of second and third numbers, we get 4. Find the numbers using matrices.Solution:**

Let the three numbers be x, y and z. According to the given conditions,

x + y + z = 2

x + z – 2y = 8, i.e., x – 2y + 2 = 8

and y + z + 3x = 4, i.e., 3x + y + z = 4

Hence, the system of linear equations is

x + y + z = 2

x – 2y + z = 8

3x + y + z = 4

These equations can be written in the matrix form as :

By equality of matrices,

x + y + z = 2 ……(1)

-3y = 6 ……(2)

– 2y – 2z = -2 ……..(3)

From (2), y = -2

Substituting y = -2 in (3), we get,

-2(-2) – 2z = -2

∴ -2z = -6 ∴ z = 3

Substituting y = -2, z = 3 in (1), we get,

x – 2 + 3 = 2 ∴ x = 1

Hence, the required numbers are 1, -2 and 3.

**Question 5.The total cost of 3 T.V. sets and 2 V.C.R.s is ₹ 35000. The shop-keeper wants profit of ₹ 1000 per television and ₹ 500 per V.C.R. He can sell 2 T. V. sets and 1 V.C.R. and get the total revenue as ₹ 21,500. Find the cost price and the selling price of a T.V. sets and a V.C.R.Solution:**
Let the cost of each T.V. set be ₹ x and each V.C.R. be ₹ y. Then the total cost of 3 T.V. sets and 2 V.C.R.’s is ₹ (3x + 2y) which is given to be ₹ 35,000.

∴ 3x + 2y = 35000

The shopkeeper wants profit of ₹ 1000 per T.V. set and of ₹ 500 per V.C.R.

∴ the selling price of each T.V. set is ₹ (x + 1000) and of each V.C.R. is ₹ (y + 500).

∴ selling price of 2 T.V. set and 1 V.C.R. is

₹ [2(x + 1000) + (y + 500)] which is given to be ₹ 21,500.

∴ 2(x + 1000) + (y + 500) = 21500

∴ 2x + 2000 + y + 500 = 21500

∴ 2x + y = 19000

Hence, the system of linear equations is

3x + 2y = 35000

2x + y = 19000

These equations can be written in the matrix form as :

By equality of matrices,

2x + y = 19000 ……….(1)

-x = -3000 ……….(2)

From (2), x = 3000

Substituting x = 3000 in (1), we get,

2(3000) + y = 19000

∴ y = 13000

∴ the cost price of one T.V. set is ₹ 3000 and of one V.C.R. is ₹ 13000 and the selling price of one T.V. set is ₹ 4000 and of one V.C.R. is ₹ 13500.