if theta is an angle in the 1st quadrant& cos theta= 1/2 find value of cos 2theta?

15 ⤊

2006-12-07 09:28:45 · 2 answers · asked by MSI 1 in Science & Mathematics ➔ Mathematics

cos theta=.5
theta=arc cos .5=60°
2 theta=120°
cos 120°=-.5

2006-12-07 09:33:44 · answer #1 · answered by yupchagee 7 · 15⤊ 0⤋

cos 2 theta = cosÂ² theta - sinÂ² theta = cosÂ² theta - (1-cosÂ² theta)
= 2 cosÂ² theta -1 = 2*1/4 -1 = -1/2.

2006-12-07 17:33:31 · answer #2 · answered by steiner1745 7 · 0⤊ 0⤋