View Full Version : Trigonometric Identities.
Goldenboko
10-26-2008, 05:28 PM
Still trying to perfect these, I ain't too good at them yet.
How do I show that (1+cos#)(1-cos#)=sin^2#
I get to 1-cos^2#=sin^2#
But then I forget what to do :o
qwertysaur
10-26-2008, 05:35 PM
1-cos<sup>2</sup> = sin<sup>2</sup>
Goldenboko
10-26-2008, 05:35 PM
Seriously? I was hoping it was that easy! xD
I might be back for more help in a bit xD
rubah
10-26-2008, 06:01 PM
don't forget about sec^2=1 + tan^2!
Okay, Boko. We get it. You're smart. :mad2:
Goldenboko
10-26-2008, 06:09 PM
If I was smart I wouldn't be asking for help Yar-Yars!
qwertysaur
10-26-2008, 06:18 PM
don't forget about sec^2=1 + tan^2!
Yeah, there are 5 that you absolutely have to memorize, but 4 of them are derivable from the first
sin<sup>2</sup> + cos<sup>2</sup> = 1
subtract sin<sup>2</sup> to get
cos<sup>2</sup> = 1 - sin<sup>2</sup>
subtract cos<sup>2</sup> to get
sin<sup>2</sup> = 1 - cos<sup>2</sup>
divide by sin<sup>2</sup> to get
1 + cot<sup>2</sup> = csc<sup>2</sup>
divide by cos<sup>2</sup> to get
1 + tan<sup>2</sup> = sec<sup>2</sup>
Just wait until you get the double angles, half angles and sum and difference equations mixed in there. You will have oh so much fun XD
rubah
10-26-2008, 07:14 PM
can you post the double angle and half angle formulas? I always forget them.
Tavrobel
10-26-2008, 07:16 PM
Double and Half Angles. (http://www.sosmath.com/trig/douangl/douangl.html)
Flying Mullet
10-26-2008, 07:24 PM
This is why I like teachers who will let you have a cheat sheet with these identities on tests.
Goldenboko
10-26-2008, 07:26 PM
This is why I like teachers who will let you have a cheat sheet with these identities on tests.
My teacher wants me to memorize them. :(
rubah
10-26-2008, 07:30 PM
sadly, calculus teachers are not some of those teachers :'(
Double Angle Theorem (typed up from tav's link):
cos(2x )= cos<sup>2</sup>(x) - sin<sup>2</sup>(x)
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