22(sin^2 9-cos^2 9)/cos 18

Question

22(sin^2 9-cos^2 9)/cos 18

Answer 1

Sin^2 9 - cos^2 9 = -(cos^2 9 - sin^2 9) = -(cos(9 + 9) * cos(9 - 9) =
= -(cos18 * cos0) = -(cos18 * 1) = -cos18.

22 (-cos18) / cos18 = -22 .