Question

Соs^4 15^0-sin^4 15^0 вычислите

Answer 1

$\cos^415а-\sin^415а=(\cos^215а-\sin^215а)(\underbrace{\cos^215а+\sin^215а}_{=1})=\\ \\ =\cos^215а-\sin^215а=\cos(2\cdot15а)=\cos30а=\frac{\sqrt{3}}{2}$