Question

1×2+2×3+3×4....10×11

Answer 1

hope it may help you,actually it is not AP,GP, the difference between two elements are 4,6,8,10...

Answer 2

sigma(n*(n+1)) where n=1 to 10
i.e sigma(n^2)+sigma(n) where n=1 to 10
= n(n+1)(2n+1)/6 + n(n+1)/2
= 10(11)(21)/6 + 10(11)/2
=440
Hence Ans: 440.