Cho các số thực \(\mathrm{a}, \mathrm{b}, \mathrm{c}\) thỏa mãn \(c^{2}+a=18\) và \(\lim _{x \rightarrow+\infty}\left(\sqrt{a x^{2}+b x}-c x\right)=-2\). Tính giá trị biểu thức \(P=a+b+5 c\).
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Phương pháp giải
Dạng vô định \(\infty-\infty\)
Lời giải
\(\lim _{x \rightarrow+\infty}\left(\sqrt{a x^{2}+b x}-c x\right)=\lim _{x \rightarrow+\infty} \frac{\left(a-c^{2}\right) \cdot x^{2}+b x}{\sqrt{a x^{2}+b x}+c x}=\lim _{x \rightarrow+\infty} \frac{\left(a-c^{2}\right) \cdot x+b}{\sqrt{a+\frac{b}{x}}+c}=-2\)
Khi và chỉ khi \(\left\{\begin{array}{l}a-c^{2}=0 \frac{b}{\sqrt{a}+c}=-2\end{array} \Leftrightarrow\left\{\begin{array}{l}a=c^{2} b=-2 \sqrt{a}-2 c\end{array}\right.\right.\).
Kết hợp với \(c^{2}+a=18\)
Khi đó \(2 c^{2}=18 \Leftrightarrow c^{2}=9 \rightarrow a=9\) và \(c=3\) (vì \(c \neq-\sqrt{a}\) )
Vậy \(b=-2 \sqrt{a}-2 c=-2 \sqrt{9}-2.3=-12\) nên \(a+b+5 c=9-12+5.3=12\).
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