S=1002−992+982−972+........................+22−12; what is the value of S?
5050
(1002−992)+(982−972)+..................+(22−12)
= 199 + 195 + 191 + 187 + ......................... + 3
=(199+3)×502=5050.
Even simpler way of calculation would be:
(100+99)+(98+97)+...+(2+1)
= Sum of first 100 natural numbers =100×1012=5050