## MP Board Class 7th Maths Solutions Chapter 12 Algebraic Expressions Ex 12.3

Question 1.
If m = 2, find the value of:
(i) m – 2
(ii) 3m – 5
(iii) 9 – 5m
(iv) 3m2 – 2m – 7
(v) $$\frac{5 m}{2}$$ – 4
Solution:
(i) m – 2 = 2 – 2 = 0
(ii) 3m – 5 = (3 × 2) – 5 = 6 – 5 = 1
(iii) 9 – 5m = 9 – (5 × 2) = 9 – 10 = -1
(iv) 3m2 – 2m – 7 = 3 × (2 × 2) – (2 × 2) – 7
= 12 – 4 – 7 = 1
(v) $$\frac{5 m}{2}$$ – 4 = $$\left(\frac{5 \times 2}{2}\right)$$ – 4 = 5 – 4 = 1

Question 2.
If p = -2, find the value of:
(i) 4p + 7
(ii) -3p2 + 4p + 7
(iii) -2p3 – 3p2 + 4p + 7
Solution:
(i) 4p + 7 = 4 × (-2) + 7 = -8 + 7 = -1
(ii) -3p2 + 4p + 7 = -3 × (-2) × (-2) + 4 × (-2) + 7 = -12 – 8 + 7 = -13
(iii) -2p3 – 3p2 + 4p + 7
= -2 × (-2) × (-2) × (-2) – 3 × (-2) × (-2) + 4 × (-2) + 7
= 16 – 12 – 8 + 7 = 3

Question 3.
Find the value of the following expressions, when x = -1:
(i) 2x – 7
(ii) -x + 2
(iii) x2 + 2x + 1
(iv) 2x2 – x – 2
Solution:
(i) 2x – 7 = 2 × (-1) – 7 = -2 – 7 = -9
(ii) – x + 2 = – (-1) + 2 = 1 + 2 = 3
(iii) x2 + 2x + 1 = (-1) × (-1) + 2 × (-1) + 1 = 1 – 2 + 1 = 0
(iv) 2x2 – x – 2 = 2 × (-1) × (-1) – (-1) – 2 = 2 + 1 – 2 = 1 Question 4.
If a = 2, b = -2, find the value of:
(i) a2 + b2
(ii) a2 + ab + b2
(iii) a2 – b2
Solution:
(i) a2 + b2 = 2 × 2 + (-2) × (-2) = 4 + 4 = 8
(ii) a2 + ab + b2 = (2 × 2) + 2 × (-2) + (-2) × (-2) = 4 – 4 + 4 = 4
(iii) a2 – b2 = 2 × 2 – (-2) × (-2) = 4 – 4 = 0

Question 5.
When a = 0, b = -1, find the value of the given expressions:
(i) 2a + 2b
(ii) 2a2 + b2 + 1
(iii) 2a2b + 2ab2 + ab
(iv) a2 + ab + 2
Solution:
(i) 2a + 2b = 2 × (0) + 2 × (-1) = 0 – 2 = – 2
(ii) 2a2 + b2 + 1 = 2 × (0) × (0) + (-1) × (-1) + 1 = 0 + 1 + 1 = 2
(iii) 2a2b + 2ab2 + ab = 2 × (0) × (0) × (-1) + 2 × (0) × (-1) × (-1) + 0 × (-1)
= 0 + 0 + 0 = 0
(iv) a2 + ab + 2 = (0) × (0) + 0 × (-1) + 2
= 0 + 0 + 2 = 2

Question 6.
Simplify the expressions and find the value, if x is equal to 2.
(i) x + 7 + 4 (x – 5)
(ii) 3(x + 2) + 5x – 7
(iii) 6x + 5(x – 2)
(iv) 4(2x – 1) + 3x+ 11
Solution:
(i) x + 7 + 4(x – 5) = x + 7 + 4x – 20
= (1 + 4)x + (7 – 20) = 5x – 13
Putting x = 2 we get,
5x – 13 = (5 × 2) – 13 = 10 – 13 = -3

(ii) 3 (x + 2) + 5x – 7 = 3x + 6 + 5x – 7
= (3 + 5)x + (6 – 7) = 8x – 1
Putting x = 2 we get,
8x – 1 = (8 × 2) – 1 = 16 – 1 = 15

(iii) 6x + 5(x – 2) = 6x + 5x – 10
= (6 + 5)x – 10 = (11x – 10)
Putting x = 2 we get,
11x – 10 = (11 × 2) – 10 = 22 – 10 = 12

(iv) 4(2x – 1) + 3x + 11 = 8x – 4 + 3x + 11
= (8 + 3) × + (11 – 4) = 11x + 7
Putting x = 2 we get,
11x + 7= (11 × 2) + 7 = 22 + 7 = 29

Question 7.
Simplify these expressions and find their values if x = 3, a = -1, b = -2.
(i) 3x – 5 – x + 9
(ii) 2 – 8x + 4x + 4
(iii) 3a + 5 – 8o + 1
(iv) 10 – 3b – 4 – 5b
(v) 2a – 2b – 4 – 5 + a
Solution:
(i) 3x – 5 – x + 9 = (3 – 1) x + (-5 + 9)
= 2x + 4 = (2 × 3) + 4 [∵ x = 3]
= 6 + 4 = 10

(ii) 2 – 8x + 4x + 4 = 2 + 4 + (- 8 + 4)x
= 6 – 4x = 6 – (4 × 3) = 6 – 12 = -6 [∵ x = 3]

(iii) 3a + 5 – 8a +1 = (3 – 8)a + (5 + 1)
= -5a + 6
= -5 × (-1) + 6     [∵ a = -1]
= 5 + 6 = 11

(iv) 10 – 3b – 4 – 5b = 10 – 4 + (- 3 – 5)b
= 6 – 8b = 6 – 8 × (-2) [∵ b = -2]
= 6 + 16 = 22

(v) 2a – 2b – 4 – 5 + a = (2 + 1)a – 2b – 4 – 5
= 3a – 2b – 9
= 3 × (-1) – 2 × (-2) – 9 [∵ a = -1, b = -2]
= -3 + 4 – 9 = -8

Question 8.
(i) If z = 10, find the value of z3 – 3(z – 10).
(ii) If p = -10, find the value of p2 – 2p -100.
Solution:
(i) For z = 10,
z3 – 3z + 30
= (10 × 10 × 10) – (3 × 10) + 30
= 1000 – 30 + 30 = 1000

(ii) For p = -10,
p2 – 2p – 100
=(-10) × (-10) – 2 × (-10) – 100
= 100 + 20 – 100 = 20 Question 9.
What should be the value of a if the value of 2x2 + x – a equals to 5, when x = 0?
Solution:
When x = 0; 2x2 + x – a = 5,
∴ (2 × 0) + 0 – a = 5
⇒ 0 – a = 5
⇒ a = -5

Question 10.
Simplify the expression and find its value when a = 5 and b = -3.
2(a2 + ab) + 3 – ab
Solution:
2(a2 + ab) + 3 – ab = 2a2 + 2ab + 3 – ab
= 2a2 + (2 – 1) ab + 3 = 2a2 + ab + 3
= 2 × (5 × 5) + 5 × (-3) + 3       [ ∵ a = 5, b = – 3]
= 50 – 15 + 3 = 38