Well... what have you attempted so far?
At least in my opinion, this is not a "do your homework for you" forum, so please show some of your work or thought process, be it correct or not.
Anyhow, here are some hints:
(a) If you get stuck at proving identities, you can always try to prove "LHS-RHS=0" instead.
In the solution below, the method of completing square is used, but we're adding/subtracting the middle term instead of the usual last term.
If you've tried the "LHS-RHS=0" approach, you might get an idea why I chose to complete square
(or I've been factoring x^4+y^4 way too many times lately).
(b) Just repeated usage of sinx=cos(pi/2-x).
FULL SOLUTION, STRONGLY ADVICE YOU ATTEMPT THE PROBLEM BEFORE VIEWING
https://www.docdroid.net/4nzV7H4/document-pdf
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本帖最後由 風之男 於 2021-1-7 06:12 PM 編輯 ]