Question

4cos2a = ?
cos a = 2/5

Answer 1

Ответ:

$\displaystyle-\frac{68}{25}$

Объяснение:

$\displaystyle 4\cos 2a=4(\cos^2a-\sin^2a)=4(\cos^2a-(1-\cos^2a))=4(\cos^2a+\cos^2a-1)=4(2cos^2a-1)=8\cos^2a-4=8*\left(\frac25\right)^2-4=8*\frac4{25}-4=\frac{32}{25}-\frac{100}{25}=-\frac{68}{25}$