Question

Доказать тождество
1/8-cos^2a+cos^4a=1/8*cos4a

Answer 1

$\frac{1}{8}-cos^2(a)+cos^4(a)= \frac{cos(4a)}{8}\\ \frac{1}{8} -sin^2(a)cos^2(a)= \frac{cos(4a)}{8} \\1-8sin^2(a)cos^2(a)=cos(4a)\\cos(4a)=cos(4a)$