To prove cos 2a = 2 cos^2 a - 1
What do I do from cos^2 a -sin^2 a

Question

asnaseer · Answer

use the identity:$$\cos^2(a)+\sin^2(a)=1$$

mathstudent55 · Answer

$  \sin^2 a + \cos^2 a = 1$

$ \cos 2a = 2 \cos^2 a - 1 $

$ \cos 2a = 2 \cos^2 a - (\sin^2 a + \cos^2 a) $

$ \cos 2a = 2 \cos^2 a - \sin^2 a - \cos^2 a $

$ \cos 2a = \cos^2 a - \sin^2 a $

anonymous · Answer

Ok so then I get to 1 - 2 sin^2 a 
got it thanks again!

asnaseer · Answer

yw :)

asnaseer · Answer

you can also get it in terms of cos(a)

anonymous · Answer

Thank you mathstudent too

mathstudent55 · Answer

wlcm

anonymous · Answer

awesome both very helpful :) Trying to learn Precalc in a week >.< found trig was the hardest part lol

asnaseer · Answer

I'm sure you'll get the hang of it soon - practice makes perfect! :)

anonymous · Answer

Luckily I have a ton of textbooks to use, just need to see some parts explained and I usually pick it up

asnaseer · Answer

thats great - the important point here is that you have the right spirit to learn the subject matter - a rare quality that you should be proud of! :)

anonymous · Answer

Haha thanks :) it's because I got moved up to Calc instead of Precalc this year for some reason and I dont want to get dropped and be bored in class for another year with over 100% xD

asnaseer · Answer

:)