Pre-Calculus Trigonometry Eqns & Ident's: Find cos(A-B) given tanA=-1/4, A in QII...

[Macmon]

Pre-Calculus Trigonometry Eqns & Ident's: Find cos(A-B) given tanA=-1/4, A in QII...

I'm brand new to these forums so sorry if I'm putting this question into the wrong group but, anyways here is the initial question.

Find the exact value of cos(A-B) given tanA=-1/4 if Angle A is in quadrant 2 and cosB=2/5 if angle B is in quadrant 4.

So what I've done so far, isn't very much, I've drawn out a graph showing the different quadrants (All, Sin, Tan, Cos) and drawn where the two angles are located. I really don't know how to go forward with this question so any help would be appreciated.

There's also a part B) but this one isn't as important.

If cos(theta)=-3/5 and (theta) is in quadrant 3. Evaluate sin2(theta)

Again thanks to anyone who is willing to help!

 

[Deleted member 4993]

I'm brand new to these forums so sorry if I'm putting this question into the wrong group but, anyways here is the initial question.

Find the exact value of cos(A-B) given tanA=-1/4 if Angle A is in quadrant 2 and cosB=2/5 if angle B is in quadrant 4.

So what I've done so far, isn't very much, I've drawn out a graph showing the different quadrants (All, Sin, Tan, Cos) and drawn where the two angles are located. I really don't know how to go forward with this question so any help would be appreciated.

There's also a part B) but this one isn't as important.

If cos(theta)=-3/5 and (theta) is in quadrant 3. Evaluate sin2(theta)

Again thanks to anyone who is willing to help!

Hint:

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

 

[Macmon]

Thanks! That really helped!