Let S1=n∑K=1K,S2=n∑K=1K2 and S3=n∑K=1K3, then S41S22−S22S23S21+S23 is equal to
If Sk=1+2+3+4+...+kk, find the value of S21+S22+S23+...+S2n.
Also, determine ∑(Snn).