Question

Evaluate 6²-7²+8²-9²+..+998²-999²​

Answer 1

Answer:

-499,485.

Step-by-step explanation:

We can transform this to an arithmetic series by  working it out in pairs:

6^2 - 7^2 = (6-7)(6+7) = -13

8^2 - 9^2 = (8-9)*8+9) = -17  

10^2 - 11^2 = -1 * 21 = -21 and so on

The common difference is -4.

The number of terms in this series  is (998 - 6) / 2 + 1

= 992/2 + 1 = 497.

Sum of n terms of an A.S:

= n/2 [2a1 + (n - 1)d  

Here a1 = -13, n = 497, d = -4:

Sum = (497/2)[-26 - 4(497-1)]

= 497/2 * -2010

= -499,485.