SOLUTION: This question is ridiculously confusing to me, any help is greatly appreciated! If it is known that sin a = 4/5, pi/2 < a < pi and that sin b =

SOLUTION: This question is ridiculously confusing to me, any help is greatly appreciated! If it is known that sin a = 4/5, pi/2 < a < pi and that sin b = - (2 * sqrt(5) / 5), pi < b <

Question 868504: This question is ridiculously confusing to me, any help is greatly appreciated!
If it is known that sin a = 4/5, pi/2 < a < pi
and that sin b = - (2 * sqrt(5) / 5), pi < b < 3pi/2,
find the exact value of cos( a + b )
Pretty please help me out, i'm in a crunch right now :/ thanks though :)
Answer by Fombitz(32388) (Show Source): You can put this solution on YOUR website!
From trig relation,

So you need to find and .
Since
then,

Since you're given information that , then must be negative (2nd quadrant) so you choose,

Now for .
Same thing,

From the information, lies in the 3rd quadrant, where the cosine is also negative

Now go back to the original equation and plug in the values.

.
.
.
As a check using inverse trig functions,
degrees
degrees
degrees

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