1.2.3.+2.3.5+3.4.7+...to n terms.

Answer 1

time to do some thinking, using what you already know. Your sum is

∑k(k+1)(2k+1) = 2k^3 + 3k^2 + k
= 2∑k^3 + 3∑k^2 + ∑k

These are just special sums whose formulas you already know.