what is the trick to use trignometric identities in proves of trignometric problems????????
It depends on the proof at hand.
it's always helpful to keep the RHS in mind if you are starting with the LHS of the statement (sometimes it is easier to start from RHS and prove it to be equal to LHS).
Use the identities which will help you to make the expression closer and closer to what you want to prove.
Hope this helps.
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