Question

(2+4+6+.......+78)-(1+3+5+......+57)

Answer 1

2 x ( 1 + 2 + 3 + .... + 39) - (2x 0 + 1 + 2 x 1 + 1 + 2x 2+1 + ... + 2 x 28+1) =
= 2 x 39 x 40 : 2 -[2 x (0+1+2+...28) + 29] =
= 39 x 40 - 2 x 28 x 29:2 - 29 =
= 39 x 40 - 28 x 29 - 29 =
= 1560 - 783 = 777