Question

Найдите 147cos2a,если cos a=3/7

Answer 1

$147Cos2\alpha=147(2 Cos^{2} \alpha-1)=147[2*( \frac{3}{7}) ^{2}-1]=147(2* \frac{9}{49}-1)=147*( \frac{18}{49}-1)=147*(- \frac{31}{49})=-31*3= -93$