Question

Which of the following is not true?
(7 + 8) + 9 = 7 + (8 + 9)
(7×8) × 9 = 7 × (8 x 9)
7 + 8×9 = (7 + 8) X (7 + 9)
7 x (8 + 9) = (7 x 8) + (7 x 9)
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Answer 1

$\huge\bold{Answer:-}$

d) 7 × ( 8 + 9 ) = ( 7 × 8 ) ( 7 × 9 ) is not true!

Explanation :

a) ( 7 + 8 ) + 9 = 7 + ( 8 + 9 )

15 + 9 = 7 + 17

24 = 24

LHS = RHS

Therefore, it's true!

b) ( 7 × 8 ) × 9 = 7 × ( 8 × 9 )

56 × 9 = 7 × 72

504 = 504

LHS = RHS

Therefore, it's true!

c) 7 + 8 × 9 = ( 7 + 8 ) × ( 7 + 9 )

7 + 72 = 15 × 16

79 = 240

LHS is not equal to RHS.

Therefore, it's not true!

d) 7 × ( 8 + 9 ) = ( 7 × 8 ) ( 7 × 9 )

7 × 17 = 56 + 63

119 = 119

LHS = RHS

Therefore, it's true!

Answer 2

Question :-

which of the following is not true ?

a) (7+8)+9=7+(8+9)

b) (7×8)×9=7×(8×9)

c) 7+8×9=(7+8)×(7+9)

d) 7×(8+9)=(7×8)+(7×9)

Solution :-

how to find it ?

here we have to prove that lhs is equal to RHS then we can find all the solutions and which is not true .

Answer :-

a) (7+8)+9=7+(8+9)

=> 15+9=7+17

=> 24=24

So, LHS =RHS

b) (7×8)×9=7×(8×9)

=> 56×9=7×72

=> 504=594

So, Here also LHS = RHS

c) 7+8×9=(7+8)×(7+9)

=> 7+72=15×16

=>79 = 240

Here, the LHS is not equal to the RHS .

d) 7×(8+9)=(7×8)+(7×9)

=> 7×17=56+63

=>119=119

So, here LHS = RHS

Answer :-

in the given question "c" option is the only where the LHS is not equal to RHS . SO, it is not correct .

Hence, c is wrong