Tính nhanh: 135^2 + 20^2 - 15^2 - 40.135

* Hướng dẫn giải

$\begin{array}{l}{135}^2+{20}^2-{15}^2-40.135\\ =\left({135}^2-\mathrm{2.135.20}+{20}^2\right)-{15}^2\\ ={\left(135-20\right)}^2-{15}^2\\ ={115}^2-{15}^2=\left(115+15\right)\left(115-15\right)\\ =130.100=13000\end{array}$