SOLUTION: I have a Russian Trig teacher and could not understand this lesson. We were suppose to prove the identities. The problem is: Prove cos(810degrees-x)=sinx He was suppose to g

Question 932116: I have a Russian Trig teacher and could not understand this lesson. We were suppose to prove the identities.
The problem is:
Prove cos(810degrees-x)=sinx
He was suppose to give us the formula for it, but never did. I think if you can just explain this first one I'll be able to figure out the rest.
Thanks!
Answer by stanbon(75887) (Show Source): You can put this solution on YOUR website!
Prove cos(810degrees-x)=sinx
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You can always throw away multiples of 360 degrees.
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Your problem::
810-2*360 = 90 degrees
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So, cos(810 - x) = cos(90-x) = sin(x)
because cos and sin are complementary trig functions.
Cheers,
Stan H.
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