Question

11^2+12^2+13^2+14^2+......+20^2 = ?

Answer 1

(1^2+2^2+3^2......20^2)-(1^2+2^2+3^2+.....10^2)
we know ,
this series
Sn=n(n+1)(2n+1)/6
use this
Sn=S20-S10
=20(21)(41)/6-10(11)(21)/6
=2485