Question

Сократите дробь 3x-9/x^2+4-21. Срочно!

Answer 1

$x^2+4x-21=0\\D=16-4\cdot(-21)=16+84=100\\x_{1,2}=\frac{-4\pm10}2\\x_1=-7,\;x_2=3\\\\\frac{3x-9}{x^2+4x-21}=\frac{3(x-3)}{(x+7)(x-3)}=\frac3{x+7}$