Question
What is cos 2theta for sin theta = -2/13 in 180 degs < theta < 270 degs?
Answers
Reiny
sinØ = -2/13 and we are told Ø is in III
so y = -2, r = 13
x^2 + y^2 = r^2
x^2 + 4 = 169
x= ±√165 , but in III x = -√165
cosØ = -√165/13
cos 2Ø = cos^2 Ø - sin^2 Ø
= 165/169 - 4/169
= 161/169
so y = -2, r = 13
x^2 + y^2 = r^2
x^2 + 4 = 169
x= ±√165 , but in III x = -√165
cosØ = -√165/13
cos 2Ø = cos^2 Ø - sin^2 Ø
= 165/169 - 4/169
= 161/169