Bài 4:
c: \(C=\sin\left(\pi+a\right)-cos\left(\frac{\pi}{2}-a\right)+\cot\left(2\pi-a\right)+\tan\left(\frac32\pi-a\right)\)
=-sina-sina-cota+cota
=-2sina
d: \(D=\sin\left(9\pi+a\right)+4\cdot cos\left(\frac92\pi-a\right)+2\cdot\cot\left(a-\frac72\pi\right)\cdot\tan\left(a+\frac{\pi}{2}\right)\)
\(=\sin\left(8\pi+\pi+a\right)+4\cdot cos\left(4\pi+\frac{\pi}{2}-a\right)+2\cdot\cot\left(a-\frac{\pi}{2}-3\pi\right)\cdot\tan\left(a+\frac{\pi}{2}\right)\)
\(=\sin\left(\pi+a\right)+4\cdot cos\left(\frac{\pi}{2}-a\right)+2\cdot\cot\left(a-\frac{\pi}{2}\right)\cdot\tan\left(a+\frac{\pi}{2}\right)\)
=-sina+4*sina+2*(-tana)*(-cota)
=3*sina+2
