# Sum and difference identities help?

#### bestgio

The following relationship is known to be true for two angles A and B:

cos(A)cos(B)-sin(A)sin(B)=0.957269

Express A in terms of the angle B. Work in degrees and report numeric values accurate to 2 decimal places.

So I'm pretty lost on how to even begin this problem. I do know the sum and difference identities such as cos(a+b)=cos(a)cos(b)-sin(a)sin(b)

Any help is greatly appreciated!

#### HallsofIvy

MHF Helper
So from that identity you know that $$\displaystyle cos(A+ B)= 0.957269$$. The rest is easy- what is A+ B equal to? (The problem says "express A in terms of angle B". You are not asked to find numerical values for A and B separately- and you can't.)

#### bestgio

I tried taking inverse cosine of 0.957269 and got about 16.81 degrees since we're asked to work in degrees. I typed that into the math site and it doesn't like my answer?

#### bestgio

I'm sorry I'm still confused with what you're tying to say,
I tried taking inverse cosine of 0.957269 and got about 16.81 degrees since we're asked to work in degrees. I typed that into the math site and it doesn't like my answer

#### romsek

MHF Helper
I'm sorry I'm still confused with what you're tying to say,
I tried taking inverse cosine of 0.957269 and got about 16.81 degrees since we're asked to work in degrees. I typed that into the math site and it doesn't like my answer
so $A+B=16.81^\circ$

$A=16.81^\circ - B$

did you type the last bit in or just the $16.81^\circ$ ?