



 
Michael, General "big hammer" approach to prove something is a trig identity, assuming it only involves trig functions of x,2x,3x...
General approach to proving something is NOT a trig identity: find one value of x for which it is not true. Of course, a really cunning problemsetter can find nonidentities that are true for 0,30,45,60,and 90 degrees but not in between! Good hunting! Michael, I want to add something to Robert's list.
Harley
 


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