+118625 20
E 2k(k-1) This is the entire equation
K=1
I like your make shift use of symbols. I know exactly what you mean :))
\(\displaystyle\sum_{k=1}^{20}\;\;2k(k-1)\;\;\\~\\ =2\left[\displaystyle\sum_{k=1}^{20}\;\;k^2\quad-\quad \displaystyle\sum_{k=1}^{20}\;\;k\right]\;\;\\~\\ =2\left[\frac{1}{6}*20*(20+1)*(2*20+1)\quad-\quad \frac{20}{2}(1+20)\right]\;\;\\~\\ =2\left[\frac{1}{6}*20*21*41\quad-\quad 10*21\right]\;\;\\~\\ =2\left[\frac{1}{1}*10*7*41\quad-\quad 210\right]\;\;\\~\\ =5320\)
I did not know the formula for the sum of squares, I looked it up here
https://www.wolframalpha.com/input/?i=formula+for+sum+of+n%5E2
