SOLUTION: I need help to prove the identity: Sin10y-sin4yścos4y+cos10y=tan3y

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Question 926756: I need help to prove the identity:
Sin10y-sin4yścos4y+cos10y=tan3y

Answer by Edwin McCravy(20059) About Me  (Show Source):
You can put this solution on YOUR website!
To prove the identity:

%28sin%2810y%29-sin%284y%29%29%2F%28cos%284y%29%2Bcos%2810y%29%29%22%22=%22%22tan%283y%29

The average of 10y and 4y is 7y, so write 10y as 7y+3y and 4y as 7y-3y


%28sin%287y%2B3y%29-sin%287y-3y%29%29%2F%28cos%287y-3y%29%2Bcos%287y%2B3y%29%29

Use the double angle formulas for sin(A±B) and cos(A±B):













sin%283y%29%2Fcos%283y%29

tan%283y%29

Edwin