Question 932116
Prove cos(810degrees-x)=sinx 
------
You can always throw away multiples of 360 degrees.
----
Your problem::
810-2*360 = 90 degrees
---
So, cos(810 - x) = cos(90-x) = sin(x) 
because cos and sin are complementary trig functions.
Cheers,
Stan H.
-------------