Question

Найдите 9cos2a, если cosa=1/3

Answer 1

9*cos2α=9*(2*cos²α-1)=18*cos²α-9
$cos alpha = frac{1}{3} 9cos2 alpha =18*( frac{1}{3} ) ^{2} -9=18* frac{1}{9} -9=2-9=-7$