Verifying trig identity? Sin(x+y) + Sin(x-y) = 2sinxcosy

[trighelpp]

Okay, I've been getting some of these, but I can't seem to verify this identity... any help? Here's the problem

Sin(x+y) + Sin(x-y) = 2sinxcosy

Okay, I've been working on the left side, and distribute, getting:
Sinx + Siny + Sinx - Siny

And, the sinx's add up to the 2sinx that I need for the right side, but the siny's cancel out if I don't change them around. So I changed one of them to 1/cscy... but I can't seem to work with that and the other siny to end up with cosy.

Where did I go wrong, or where do I go with it now?

 

[Deleted member 4993]

Re: Verifying trig identity?

Okay, I've been getting some of these, but I can't seem to verify this identity... any help? Here's the problem

Sin(x+y) + Sin(x-y) = 2sinxcosy

Okay, I've been working on the left side, and distribute, getting:
Sinx + Siny + Sinx - Siny <<<< Nooooooo......

\(\displaystyle \sin(\theta + \phi) \, = \, \sin(\theta)\cdot \cos(\phi) \, + \, \sin(\phi)\cdot \cos(\theta)\)

and

\(\displaystyle \sin(\theta - \phi) \, = \, \sin(\theta)\cdot \cos(\phi) \, - \, \sin(\phi)\cdot \cos(\theta)\)

now continue.....

 

[trighelpp]

Re: Verifying trig identity?

oh, dang, forgot about that identity!

Okay, well, adding that up it ends up as 2sinx2cosy... but I need it to be 2sinx(1)cosy.

or am I really tired and I'm thinking too out of the box to realize the entire term of 2sinxcosy consists of 2sinx's and 2cosy's?

if not, what do I switch up to drop a cosy to satisfy the right side of the equation? oh man, am i sure forgetting everything about math, lol.

 

[Deleted member 4993]

Re: Verifying trig identity?

oh, dang, forgot about that identity!

Okay, well, adding that up it ends up as 2sinx2cosy... but I need it to be 2sinx(1)cosy. <<< How did you get 2 there

or am I really tired and I'm thinking too out of the box to realize the entire term of 2sinxcosy

that means 2 (times) sin(x) (times) cos(x)