Find cos2u?

4 ⤊

given cosu=-2/7 and pie/2

2007-12-15 10:11:31 · 2 answers · asked by warsane b 1 in Science & Mathematics ➔ Mathematics

cos 2u = cos ² u - sin ² u
cos 2u = cos ² u - (1 - cos ² u)
cos 2u = 2 cos ² u - 1
cos 2u = 2 (4/49) - 1
cos 2u = 8 / 49 - 49 / 49
cos 2u = - 41/ 49

2007-12-19 05:54:29 · answer #1 · answered by Como 7 · 2⤊ 0⤋

cos2u
= 2cos^2(u) - 1
= 2(-2/7)^2 - 1
= 8/49 - 1
= -41/49

2007-12-15 18:18:24 · answer #2 · answered by sahsjing 7 · 2⤊ 0⤋