I'm studying for a final and I am stumped, I have the answer already but how the heck does one get CosXCosB - SinXSinB from Cos(X + B)?

Question

anonymous · Answer

$$\cos (\sin^{-1}\left(\begin{matrix}3 \ 5\end{matrix}\right)+\cos^{-1} \left(\begin{matrix}5 \ 13\end{matrix}\right))$$

primeralph · Answer

Well, it's an identity.

anonymous · Answer

yes

primeralph · Answer

What does your next statement have to do with your original post?

anonymous · Answer

that is the question it says find the exact value of that equation

primeralph · Answer

Is that 3/5 and 5/13?

anonymous · Answer

yes

primeralph · Answer

Okay. Now just find a relationship between the double angle formula and your problem. I've found one already. Try.

anonymous · Answer

i have a paper of all the identities i feel like a complete jackass it is listed there, i have to open my eyes lol
the identity states $$\cos (U \pm V) = cosUcosV \mp sinUsinV$$

primeralph · Answer

Hold, on. I'm helping someone else.

anonymous · Answer

thanks pal

primeralph · Answer

Okay, let's start from the top.

anonymous · Answer

its done man it was a sum and difference identity

anonymous · Answer

it was was on my sheet of ids

primeralph · Answer

So you don't need help anymore?

anonymous · Answer

no you helped me by saying it was an identity I looked at the sheet and there it was lol I wasn't looking hard enough so thank you

primeralph · Answer

Well, you're smart. Good luck.

anonymous · Answer

thanks