# Thread: Sum and Didderence Identities w/Pictures

1. ## Sum and Didderence Identities w/Pictures

I know the identities by hard. My problem is when its in fraction form with a radical. When do you multiply by conjugate and when not? (See picture for problem with tangent)

Second problem I tried to use sine of sum identity (picture with words)

2. ## Re: Sum and Didderence Identities w/Pictures

tan 195 = tan ( 180 + 15) = tan 15
use tan 15= tan(45-30)

3. ## Re: Sum and Didderence Identities w/Pictures

sin(A+B)=sinA cosB +cosA sinB

sin A= 3/5
sin B =5/13
as A is in second quadrant
therefore cos will be negative
cos A = -4/5
cos B =12/13
can u find sin (A+B) now?

5. ## Re: Sum and Didderence Identities w/Pictures

Originally Posted by mpx86
sin(A+B)=sinA cosB +cosA sinB sin A= 3/5 sin B =5/13 as A is in second quadrant therefore cos will be negative cos A = -4/5 cos B =12/13 can u find sin (A+B) now?
How did you get -4/5 and 12/13 for cosines? Cos A= -4/5 shouldnt you apply even/odd properties?

6. ## Re: Sum and Didderence Identities w/Pictures

You probably used the Sine pythag identity

7. ## Re: Sum and Didderence Identities w/Pictures

I got it now thanks.