Question

найти 9 косинус 2 альфа если косинус=1/3

Answer 1

$9cos2alpha=9(2cos^2alpha-1)=18cos^2alpha-9\ \ if~~ cosalpha=dfrac{1}{3},~~~then~~~~18cos^2alpha-9=18cdotbigg(dfrac{1}{3}bigg)^2-9=2-9=-7$