SOLUTION: Find sin(2a) when a terminates in quadrant II. cosa=-4/5. (I got -24/25, but I am not sure if I solved it correctly)

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Question 1174754: Find sin(2a) when a terminates in quadrant II. cosa=-4/5. (I got -24/25, but I am not sure if I solved it correctly)
Found 2 solutions by MathLover1, MathTherapy:
Answer by MathLover1(20850)   (Show Source): You can put this solution on YOUR website!

you got it right!

given:


when a terminates in quadrant ,
use identity:
.........substitute





then or
use identity:
..........substitute and
note: since is in quadrant , the info leads to,so we have



Answer by MathTherapy(10555)   (Show Source): You can put this solution on YOUR website!
Find sin(2a) when a terminates in quadrant II. cosa=-4/5. (I got -24/25, but I am not sure if I solved it correctly)
You are CORRECT!

This is one of the PYTHAG TRIPLES, of the 3-4-5 variety.

And, with  , OPPOSITE = y = 3 (Quadrant 2), and 

We now get:  

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