*Bài làm:
a, Ta có: \(y=f\left(x\right)=|x|\)
⇒ \(\left\{{}\begin{matrix}f\left(4\right)=y=|4|=4\\f\left(5\frac{1}{6}\right)=y=|5\frac{1}{6}|=5\frac{1}{6}=\frac{31}{6}\\f\left(-9,4\right)=y=|-9,4|=9,4\end{matrix}\right.\)
⇒ \(\left\{{}\begin{matrix}5.f\left(4\right)=5y=5.4=20\\6.f\left(5\frac{1}{6}\right)=6y=6.\frac{31}{6}=31\\8.f\left(-9,4\right)=8y=8.9,4=\text{75.2}\end{matrix}\right.\)
⇒ \(5f\left(4\right)+6f\left(5\frac{1}{6}\right)-8f\left(-9,4\right)\) = \(20+31-75.2\) = \(\text{-24.2}\)
~Vậy: \(5f\left(4\right)+6f\left(5\frac{1}{6}\right)-8f\left(-9,4\right)\) = \(\text{-24.2}\) .
b, Ta có: \(y=f\left(x\right)=|x|\)
⇒ \(|y|=|f\left(x\right)|=||x||\)
Mà \(|f\left(x\right)|=0\) (Theo đề cho).
⇒ \(|y|=|f\left(x\right)|=||x||=0\)
⇒ \(y=f\left(x\right)=|x|=0\)
⇒ \(\left|x\right|=0\)
⇒ \(x=0\) .
~Vậy: \(x=0\) thỏa mãn đề .
➤ Chúc bạn học tốt!
