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Explanation:
The rule is
If A+C = 90, then sin(A) = cos(C). We could also say cos(A) = sin(C).
I recommend drawing out the picture of what's going on. See below.
sin(A) = 8/17 has the opposite side 8 over hypotenuse 17.
cos(C) = 8/17 has 8 as the adjacent side now. The hypotenuse stays the same.
So that's why sin(A) = cos(C).
