\(\left(10^2-5^2\right)+\left(9^2-4^2\right)+..+\left(6^2-1^2\right)\)
\(\left(10-5\right)\left(10+5\right)+...+\left(6-1\right)\left(6+1\right)\)
\(5.15+5.13+5.11+5.9+5.7\)
\(5\left(15+13+11+9+7\right)\)
\(5.\frac{\left(15+7\right).5}{2}=\frac{5.22.5}{2}=11.25=25+250=275\)