Question

Which of the following is not true?

Options

• (7 + 8) + 9 = 7 + (8 + 9)

• (7 × 8) × 9 = 7 × (8 × 9)

• 7 + 8 × 9 = (7 + 8) × (7 + 9)

• 7 × (8 + 9) = (7 × 8) + (7 × 9)

Answer 1

7 + 8 × 9 = (7 + 8) × (7 + 9)

Explanation:

a. We have, (7 + 8) + 9 = 7 + (8 + 9)

LHS = (7 + 8) + 9

= 15 + 9

= 24

RHS = 7 + (8 + 9)

= 7 + 17

= 24 

LHS = RHS

So, it is true.

b. We have, (7 × 8) × 9 = 7 × (8 × 9)

LHS = (7 × 8) × 9

= 56 × 9

= 504 

RHS = 7 × (8 × 9)

= 7 × 72

= 504 

LHS = RHS

So, it is true.

c. We have, 7 + 8 × 9 = (7 + 8) × (7 + 9)

LHS = 7 + 8 × 9

= 7 + 72

= 79

RHS = (7 + 8) × (7 + 9)

= 15 × 16

= 240

∴ 79 ≠ 240

∴ LHS ≠ RHS

So, it is false.

d. We have, 7 × (8 + 9) = (7 × 8) + (7 × 9)

LHS = 7 × (8 + 9)

= 7 × 17

= 119 

RHS = (7 × 8) + (7 × 9)

= 56 + 63

= 119

∴ LHS = RHS

So, it is true.