[tex]\cos\frac{5\pi}6=\cos(\pi-\frac{\pi}6)=-\cos\frac\pi6=-\frac{\sqrt3}2\\\\ \sin\frac{7\pi}4=\sin(2\pi-\frac\pi4)=-\sin\frac\pi4=-\frac{\sqrt2}2\\\\ \cos\frac{5\pi}4=\cos(\pi+\frac{\pi}4)=-\cos\frac\pi4=-\frac{\sqrt2}2\\\\ \sin(-\frac{5\pi}3)=\sin(-\frac{5\pi}3+2\pi)=\sin\frac\pi3=\frac{\sqrt3}2[/tex]
