Question

If 90° <0< 180° and sin 0 = 2/7. find cos 20.

A-41/49
B-8/49
C8/49
D41/49

Answer 1

Answer: 41/49 (choice D)

Work Shown:

`\cos(2\theta) = 1 - 2\sin^2(\theta)\\\\\cos(2\theta) = 1 - 2\left((2)/(7)\right)^2\\\\\cos(2\theta) = 1 - 2\left((4)/(49)\right)\\\\\cos(2\theta) = 1-(8)/(49)\\\\\cos(2\theta) = (49)/(49)-(8)/(49)\\\\\cos(2\theta) = (49-8)/(49)\\\\\cos(2\theta) = (41)/(49)\\\\`