More Proving Trig Identities

[clz9368]

I have a type of problem that I have never seen before so I have no clue where to even begin.

(cos5x*cos3x)+(sin5x*sin3x)=cos^2x-sin^2x
I thought maybe you added the 5 and 3 for both to get
cos8x+sin8x
Other than that, I have no idea how to start or if that is even right, where to go next.


Another problem I don't know how to work out because I have never seen a number between the sin/cos/tan, before the X.

tan2A=2tanA/(1-tan^2A)

 

[Deleted member 4993]

I have a type of problem that I have never seen before so I have no clue where to even begin.

(cos5x*cos3x)+(sin5x*sin3x)=cos^2x-sin^2x

use

cos(A-B) = cosA*cosB + sinA*sinB


I thought maybe you added the 5 and 3 for both to get
cos8x+sin8x ....................Perish that thought

Other than that, I have no idea how to start or if that is even right, where to go next.


Another problem I don't know how to work out because I have never seen a number between the sin/cos/tan, before the X.

tan2A=2tanA/(1-tan^2A)

cos(2A) = cos[sup:3efykjfr]2[/sup:3efykjfr](A) - sin[sup:3efykjfr]2[/sup:3efykjfr](A)

sin(2A) = 2 sin(A) * cos(A)

tan(2A) = sin(2A)/cos(2A)