Question

C=1x2+2x3+3x4+...+10x11+11x12

Answer 1

3C = 1x2x3 + 2x3x(4-1) + ... + 11x12x(13-9)

3C = 1x2x3 + 2x3x4 - 1x2x3 + ... + 11x12x13 - 9x11x12

3C = 9x11x12

3C = 1188

C = 396

Answer 2

C = 1x2 + 2x3 + 3x4 + ... + 10x11 + 11x12

=> 3C = 1x2x3 + 2x3x4 - 1x2x3 + ... + 11x12x13 - 9x11x12

=> 3C = 9 x 11 x 12

=> 3C = 1188

=> C = 396