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**Saitenji****Member**- Registered: 2005-09-24
- Posts: 10

Can someone tell me the pythagorean identites that are used for trig?

So far, I know that sin² x + cos² x = 1 and cos² x = 1 - sin² x. I skipped several math classes and headed straight for AP Calculus...Everything else is pretty easy, but I can't seem to get the trig down.

A list of all the pythagorean identites would be very much appreciated.

Thanks in advance.

*Last edited by Saitenji (2005-09-29 12:39:23)*

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**mathsyperson****Moderator**- Registered: 2005-06-22
- Posts: 4,900

Those two imply each other, and they also imply:

tan² x + 1 = sec² x

1 + cot² x = cosec² x

There's also the obvious sin x / cos x = tan x and the half and double angle formulae, but I can't remember them and I don't have the textbook with me.

Why did the vector cross the road?

It wanted to be normal.

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**mikau****Member**- Registered: 2005-08-22
- Posts: 1,504

Doulbe angle identities are created by using the sum identiy's where both angles are the same. So instead of "sin(A+B)" you use "sin(B+B)" or 2b: Double the angle. You then replace B everywhere with A.

I have a page I copied out of the back of my mathbook that has most of the common identities listed. You definitly need to write them down for referance. You begin to remember them after using them long enough.

A logarithm is just a misspelled algorithm.

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**Saitenji****Member**- Registered: 2005-09-24
- Posts: 10

Yeah, I went over it more indepth today with my teacher 'cause I told her I didn't understand much. She was really nice about it and I believe that I am actually starting to remember several of these identities. Thanks for your guys' help as well though, wouldn't have been able to do my homework that night without it.

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