Question

Знайдіть cos2x, якщо
cosx =0.6.

Answer 1

$cos2x = {cos}^{2} x - {sin}^{2} x = \\ = {cos}^{2} x - (1 - {cos}^{2} x) = \\ = 2 {cos}^{2} x - 1 = \lbrace cosx = 0.6 \rbrace = \\ = 2 \times {0.6}^{2} - 1 = 0.72 - 1 = - 0.28$