Question

найдите 9cos2альфа если cocальфа=1/3

Answer 1

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