Common Trig Identity Derivations

[david martin]

I'm looking for a source online that gives the step by step derivation of common trig identities, such as sin(2theta) = 2sin(theta)cos(theta), cos(2theta) = cos²(theta)-sin²(theta), sin²(theta) = (1 - cos(2theta))/2, ect. I did do 20 minuntes or so of searching online, nothing was exactly what I was hoping for. Is there a website explaining the derivations or should I fork out the $100 for a trig text? Thanks

 

[rock.freak667]

The simplest way uses the these identities:

cos(A+B)= cosAcosB - sinAsinB

and sin(A+B) = sinAcosB + sinBcosA

then put A=B. Proving the above identities might prove to be a bit more difficult.