Question

Решите уравнение:
$frac{x+21}{x^{2}-9} - frac{x}{x+3} = 0$

Answer 1

$frac{x+21}{(x-3)(x+3)}- frac{x}{x+3}=0 \ \ frac{x+21-x(x-3)}{(x-3)(x+3)}=0 \ frac{x+21-x^2+3x}{(x-3)(x+3)}=0 \ *x^2-9 neq 0 = textgreater x neq ^+_--3\ -x^2+4x+21=0 \ D=16+4*21=16+84=100 \ \ x_1= frac{-4+10}{2}=-3$ 
$\ \ x_2 = frac{-4-10}{2}=-7$
-3∉