# Thread: Trig with a domain.

1. ## Trig with a domain.

I have a question that's restricted to a certain domain (I guess). Obviously I only give the answers that fall in there. The question goes like this:

0 < theta < pi/2 So I know it's between 0-90 degrees.

I have cos(theta) = 3/5

I have to find sin(2theta) which is 2sin(theta)cos(theta) so my little cheat sheet says. Do I just fill in 3/5 to that equation? It seems easy enough I guess. I am just confirming what I am doing is actually right. Thanks a million in advance.

2. yes, you'll substitute 3/5 for the cosine value, but what about the value for sine?

3. So there's probably another equation for it isn't there?

4. sketch a reference triangle in quad I. you know the ratio of the adjacent side over the hypotenuse = 3/5

so, what is the ratio of the opposite side over the hypotenuse for the same triangle?

5. Does this mean the r value = 5 and y = 3??

6. Originally Posted by cjh824
Does this mean the r value = 5 and y = 3??
r = 5 and x = 3

y = ?