ПОМОГИТЕ КАК РЕШАТЬ часть C?

$sin(\frac{7\pi}{2}-3x) = \frac{\sqrt{10}-\sqrt{8}}{\sqrt{20}-4}\\ sin(\frac{7\pi}{2}-3x)=\frac{(\sqrt{10}-\sqrt{8})(\sqrt{20}+4)}{4}\\ sin(\frac{7\pi}{2}-3x)=\frac{\sqrt{200}+4\sqrt{10}-\sqrt{160}-4\sqrt{8}}{4}\\ sin(\frac{7\pi}{2}-3x)=\frac{\sqrt{2}}{2}\\ -cos3x=\frac{\sqrt{2}}{2}\\ x=\frac{-\pi}{4}+\frac{2\pi*n}{3}\\ x=\frac{\pi}{4}+\frac{2\pi*n}{3}\\ x=\frac{\pi}{4}$

$sina+cosa=a\\ sin^3a+cos^3a=(sina+cosa)(sin^2a-sina*cosa+cos^2a)\\ (sina+cosa)^2=a^2\\ sina*cosa=\frac{a^2-1}{2}\\ \\ sin^3a+cos^3a=a(1-\frac{a^2-1}{2})=\frac{a-a^3}{2}$