Thread: Some more trig problems ( Cos(α + β), etc)

1. Some more trig problems ( Cos(α + β), etc)

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2. Originally Posted by SarahGr
I have one more question for you guys here's what I was given

Suppose sinα = 4/5 and tanβ= -3/4 where π/2 < α < β < π (pi)

the above inequality tells you the quadrant where α and β both reside.

Find:

Sin(α + β)

Cos(α + β)

Tan(α + β)

Sin2α

Cos2β

Tan2α

What i have been doing is figuring out the other trig functions

They gave me sinα=4/5 and tanβ=-3/4

the signs of your trig values below were all incorrect ... α and β are both quadrant II angles ... sine is positive, cosine and tangent are negative.

note the changes ...

from that i found cosα = -3/5 tanα = -3/4 and sinβ = +3/5 cosβ = -4/5

I tried problem a ... redo it

sin(α+β)=sinαcosβ+cosαsinβ

sin(α+β)=4/5 * 4/5 + 3/5 * -3/5

which gives me 7/25 but I'm not sure I am doing this right... what is the whole (π/2 < α < β < π) thing about am I supposed to be finding points on the unit circle or something? understand now? =/
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