Đáp án:
c) \(Q = 529\)
Giải thích các bước giải:
\(\begin{array}{l}
B6:\\
a)DK:x \ne \pm 5\\
b)P = \dfrac{{x - 5 + 2\left( {x + 5} \right) - 2x - 10}}{{\left( {x + 5} \right)\left( {x - 5} \right)}}\\
= \dfrac{{ - x - 15 + 2x + 10}}{{\left( {x + 5} \right)\left( {x - 5} \right)}}\\
= \dfrac{{x - 5}}{{\left( {x + 5} \right)\left( {x - 5} \right)}}\\
= \dfrac{1}{{x + 5}}\\
c)P = - 3\\
\to \dfrac{1}{{x + 5}} = - 3\\
\to - 3x - 15 = 1\\
\to x = - \dfrac{{16}}{3}\\
Thay:x = - \dfrac{{16}}{3}\\
\to Q = 9{\left( { - \dfrac{{16}}{3}} \right)^2} - 42.\left( { - \dfrac{{16}}{3}} \right) + 49 = 529
\end{array}\)
