Question

скорати дробь х^2 - 9 x^2 +4x -21

Answer 1

$displaystyle tt frac{x^2-9}{x^2+4x-21}=frac{(x-3)(x+3)}{x^2+7x-3x-21}=frac{(x-3)(x+3)}{x(x+7)-3(x+7)}=\\\ displaystyle tt =frac{(x-3)(x+3)}{(x+7)(x-3)}=bold{frac{x+3}{x+7}}$