Posted by Jon on .
2)Find the solution of sin2 theta = cos theta if 0 < theta < 180
A)30 degrees and 90 degrees
B)30 degrees and 150 degrees
C)30 degrees, 90 degrees, 150 degrees
D)0 degrees, 90 degrees, and 150 degrees
sin2 theta = cos theta
sin2 theta  1 = 0
(sin theta + 1)(sin theta  1)= 0
sin theta = 1 or sin theta = 1 which is B
I used the zero property

Trig(continued) 
Reiny,
For your earlier post for this same question at 3:17 am, I said
"for #2 I too, like Bob, was puzzled
First of all is it
sin 2θ = cos θ or sin^{2} θ = cos θ ?
for the first one
2sinθcosθ  cosθ = 0
cosθ(1sinθ  1) = 0
cosθ=0 or sinθ=1/2
so θ = 90º or θ = 30º or 150º for your domain, which is choice C "
how did you get your second line??? 
Trig(continued) 
Reiny,
line: cosθ(1sinθ  1) = 0
should have said
cosθ(2sinθ  1) = 0 
Trig(continued) 
drwls,
Please use ^ in front of exponents. Otherwise, we cannot tell sin (2theta) from (sin theta)^2.
Your first step is wrong, since cos theta is not necessarly equal to 1. Also, if sin theta were 1 or 1, why would the answer be B?
I will assume you meant to write sin (2theta) and will let x be theta, to simplify typing.
2 sin x cos x = cos x
2 sin x = 1
sin x = 1/2
x = 30 or 150 degrees.
You seem to have treated sin2theta as sin^2 theta, in your nexttolast step, but if you do that, the answer is completely different 
Trig(continued)Reiny 
Jon,
my question is sin 2è = cos è NOT sin2 è = cos è. I'm sorry, I didn't understand your question earlier.

Trig(continued)Reiny 
Reiny,
In that case the solution I just gave you above will be the correct one.
Notice that all 3 answers work in your original equation.