# Proving Trigonometric Identities

1. Apr 24, 2009

### mathnewb

1. The problem statement, all variables and given/known data
1) sin(x+y)sin(x-y)=cos^2y-cos^2x

2) tan(∏⁄4+x)+tan(∏/4-x)=2sec2x

3) cosx-cosy=-2sin(x+y/2)sin(x+y/2)

4) 2cotx-2tanx=4-2sec^2x/tanx

2. Relevant equations
all trig identities

3. The attempt at a solution
1) i understand that i should show what i have attempted but there is way too many lines of manipulating the equations and i don't have the time to write it down. i can say that for all of these identities i have changed them to cos and sin as these can be the easiest to solve and then used trig identities to make both sides look the same. i have nt had any luck with these and i was hoping you guys could help me

thx alot!

2. Apr 24, 2009

### yeongil

Is it (x+y)/2 or x+(y/2)?
Again... is it (4-2sec^2x)/tanx or 4-(2sec^2x/tanx)?
Sorry, you'll have to make time to write what you have so far if you want us to help. I'll give you one suggestion -- in #2, don't convert to cosines and sines as your first step. Too messy. Instead, use the tangent sum & difference formulas.

01

Last edited: Apr 25, 2009