Posted by Amirul on Wednesday, February 13, 2013 at 10:42am.
sin 2 theta + cos theta = 0
so, we use sin 2 theta = 2 sin theta cos theta right? but the problem is i'm stuck on (2 sin theta cos theta) + cos theta = 0. What should i do?

math(Trigonometry)  bobpursley, Wednesday, February 13, 2013 at 10:49am
cosTheta(2SinTheta+1)=0
remember ab=o? a or b or both is = zero

math(Trigonometry)  Amirul, Wednesday, February 13, 2013 at 10:56am
i see you factorised the question. But, still i can't find the solution. but how about this:
2sin theta cos theta =  cos theta
2sin theta =  cos theta/cos theta
2sin theta = 1
sin theta = 1/2
theta = sin^1(1/2)
theta = 30

math(Trigonometry)  Reiny, Wednesday, February 13, 2013 at 11:35am
from Bob's , cosØ(2sinØ + 1)=0
cosØ = 0 or sinØ = 1/2
if cosØ = 0, then Ø = 90° or 270° OR Ø = π/2 , 3π/2
if sinØ = 1/2, Ø must be in III or IV
Ø = 180+30 = 210° or 36030 = 330°
in radians Ø = 7π/6 , 11π/6
you totally ignored the fact it was 1/2
and did not take the CAST rule into consideration.

math(Trigonometry)  Amirul, Wednesday, February 13, 2013 at 12:25pm
Yeah, usually i used sinØ = 1/2 to find the quarters and skipped the negative to find theta.
"if sinØ = 1/2, Ø must be in III or IV. Hence, Ø = 180+30 = 210° or 36030 = 330°"
However, i got the same answers as yours :). Just to make sure whether it is correct or not.
Thanks for the guide bobpursley and Reiny!
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