# help with trigonometry

• Oct 23rd 2013, 11:10 AM
nova23
help with trigonometry
Can someone help explain this trigonometry equation to me with a detailed development?

sin2a - cos2a = 1-2 x cos2a

thank you!
• Oct 23rd 2013, 12:35 PM
ebaines
Re: help with trigonometry
One of the most basic identities to memorize because you will use it frequently is: $\displaystyle \sin^2a + \cos^2a = 1$. Rearrange this to get the $\displaystyle \sin^2a$ by itself, and then substitute for the $\displaystyle \sin^2a$ term in your equation - what do you get? By the way, you could also have replaced the $\displaystyle \cos^2a$ term in your equation to get a different form of the same equation, all of which are equal to $\displaystyle \cos(2a)$.
• Oct 23rd 2013, 01:52 PM
nova23
Re: help with trigonometry
thank you.
but can you or someone else explain even clearer how to develop and explain the equation, i'm new to maths and trigonometry and really need a full explanation as detailed as possible!
• Oct 23rd 2013, 02:12 PM
Plato
Re: help with trigonometry
Quote:

Originally Posted by nova23
thank you.
but can you or someone else explain even clearer how to develop and explain the equation, i'm new to maths and trigonometry and really need a full explanation as detailed as possible!

$\displaystyle \sin^2(x)=1-\cos^2(x)$ so just substitute on the LHS.