100- 99 + 98- 97 + ... + 4 - 3 +2 -1
=(100- 99) + (98- 97) + ... + (4 - 3) +(2 -1)
=1+1+...+1+1
=1.50
=50
100 - 99 + 98 - 97 + ... + 4 - 3 + 2 - 1
= ( 100 - 99 ) + ( 98 - 97 ) + ... + ( 4 - 3 ) + ( 2 - 1 )
= 1 + 1 + ... + 1 + 1
= 1 . ( 100 : 2 )
= 1 . 50
= 50