# Thread: Stupid simple sine/cosine problem

1. ## Stupid simple sine/cosine problem

Alright, so I have this right-angled triangle and I thought I did it right, but apparently I didn't, so any help would be great appreciated!

I wanted to find B,
So I just used b / SinB = c /sinC

SinB = b / c

SinB = sin^-1(38/63)

I end up with 37.1, but the answers said I wanted 52.9!
So I switched the sin^-1 for a cos^-1, and I ended up with 52.9. ):

Now my question is, why do I want to use cos instead of sin?
I really don't get it! I can't see why it's wrong to use sine since since is Opposite/Hypothenuse.

2. Originally Posted by Fnus
Alright, so I have this right-angled triangle and I thought I did it right, but apparently I didn't, so any help would be great appreciated!

I wanted to find B,
So I just used b / SinB = c /sinC

SinB = b / c

SinB = sin^-1(38/63)

I end up with 37.1, but the answers said I wanted 52.9!
So I switched the sin^-1 for a cos^-1, and I ended up with 52.9. ):

Now my question is, why do I want to use cos instead of sin?
I really don't get it! I can't see why it's wrong to use sine since since is Opposite/Hypothenuse.

based on your sketch (sure it's right?), you are correct ...

$m\angle{B} = \sin^{-1}\left(\frac{38}{63}\right) = 37.1^{\circ}$

$m\angle{A} = \cos^{-1}\left(\frac{38}{63}\right) = 52.9^{\circ}$

3. Yeah, I triple checked and more. ):

Well, thanks for clearing it up at least, because I really couldn't see what I'd done wrong.
I guess I'll just have to ask my teacher tomorrow!

Thanks a lot for replying. (: