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 360-30 = 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 360-30 = 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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