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Trignometric identites that are used in real life?

Can someone list me some Trig identites that are used to perform a particluar task or assigment ? Thanks.

Public Comments

1. Absolutely none... I feel your pain, I'm in trig, too.
2. cos^2(x) + sin^2(x) = 1 pops up in a whole bunch of things. Thats the only one. You will never EVER have to use the ridiculous things your text book asks you to prove. Some slightly more complicated ones will come into play when you do trig integration in calculus. But again, you will never use those calculus functions in anything anyhow, and if you do, you will use Maple to solve them for you.
3. Real life is passing the course you are taking. Study hard! .
4. As Paul W said, cos^2 + sin^2 = 1 is the king of trig identities. But half- and double-angle formulas are pretty useful, too, as well as cofunction and power identities (useful for turning a bunch of sines and cosines into just cosines). Really, it just depends on how much more math you're going to do in the "real world". If you're not going to be a mathematician, then cos^2+sin^2 is probably more than enough.