Question

Cho \(\frac{\pi}{2}< a< \pi\). Xđ dấu của bthuc \(M=cos\left(-\frac{\pi}{2}+a\right).tan\left(\pi-a\right)\)

Answer 1

\(\frac{\pi}{2}< a< \pi\Rightarrow\left\{{}\begin{matrix}sina>0\\cosa< 0\\tana< 0\end{matrix}\right.\)

\(M=cos\left(\frac{\pi}{2}-a\right)tan\left(\pi-a\right)=sina.\left(-tana\right)=-sina.tana=-\frac{sin^2a}{cosa}>0\)