Question

скорати дробь х^2 - 9 \ x^2 +4x -21

Answer 1

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