287 × 287 + 269 × 269 - 2 × 287 × 269 = ?

$$\eqalign{ & \left( {287 \times 287 + 269 \times 269 - 2 \times 287 \times 269} \right) \cr & = {\left( {287} \right)^2} + {\left( {269} \right)^2} - \left( {2 \times 287 \times 269} \right) \cr & = {\left( {287 - 269} \right)^2} \cr & = {\left( {18} \right)^2} \cr & = 324 \cr} $$
[∵ a² + b² - 2ab = (a - b)² ]