Question

Question 72

Observe the following.

1 + 2 - 3 + 4 + 5 - 6 - 7 + 8 - 9 = -5

Change one '-' sign as '+' sign to get the sum 9.

Answer 1

LHS = 1 + 2 - 3 + 4 + 5 - 6 - 7 + 8 - 9

= (1 + 2 - 3) + (5 + 4 - 9) - 6 - 7 + 8

= 0 + 0 - 6 - 7 + 8 = 2 - 7 = - 5

Now, we have to get the sum 9, so we replace the sign of (-) into (+) before 7.

On substituting the sign we get:

1 + 2 - 3 + 4 + 5 - 6 + 7 + 8 - 9

= (1 + 2 -3) + (4 + 5 - 9) - 6 + 7 + 8

= 0 + 0 - 6 + 15

= 9

Hence, on replacing the symbol '-' sign as '+' before 7, we get the sum 9.