Question

Find using identity 202×197

Answer 1

$202 \times 197 = (200 + 2)(200 - 3) \\apply \\ (x + a)(x - b) = {x}^{2} + (a - b)x - ab \\ where \: x = 200 \: a = 2 \: and \: b = 3 \\ = {200}^{2} + ( - 1) \times 200 - 6 \\ = 39794$