\(a,x\ne\pm5\\ b,\\ =\dfrac{1}{x+5}+\dfrac{2}{x-5}-\dfrac{2\left(x+5\right)}{\left(x+5\right)\left(x-5\right)}\\ \Leftrightarrow\dfrac{1}{x+5}+\dfrac{2}{x-5}-\dfrac{2}{x-5}=\dfrac{1}{x+5}\\ c,P=-3\\ \Leftrightarrow\dfrac{1}{x+5}=-3\Leftrightarrow x+5=\dfrac{-1}{3}\Leftrightarrow=\dfrac{-16}{3}\)
\(Q=9x^2-42x+49\\ =\left(3x\right)^2-2.3x.7+7^2=\left(3x-7\right)^2\\ Thay.x=\dfrac{-15}{3}:\\ =\left(3.\dfrac{-16}{3}-7\right)^2\\ =\left(-16-7\right)^2=\left(-23\right)^2\\ =529\)
1/x+5 + 2/x-5 - 2x+10/(x+5)(x-5)
= (x-5)+2(x+5)-2x+10/(x+5)(x-5)
= x-5+2x+10-2x+10/(x+5)(x-5)
=x-5/(x-5)(x+5)
=1/x+5
