Posted by Jonathan on .
if cosA = 4/5; sin <0; cosB = 12/13; 0<B<90, determine sin(AB)
thanks ... :)

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bobpursley,
The first pairing says that sinA is 4/5
If cosB is 12/13, then sinB is 7/13
Sin(AB)=CosASinBSinACosB, right? 
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Jonathan,
yeah that's right, can u go on?

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bobpursley,
I could, but I wont. However, I will be happy to critique your work or thinking

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bobpursley,
Error: sinA=3/5
in all cases, sin^2 Theta + cos^2 Theta=1 
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Jonathan,
well, i got:
(4/5)(12/13)  (4/5)(7/13)
therefore
(48/65)  (28/65)
therefore:
answer = (76/65) 
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Reiny,
from if cosA = 4/5; sin <0 we know that A is in the fourth quadrant and sinA = 3/5
from cosB = 12/13; 0<B<90 we know sinB = 5/13
sin(AB) = sinAcosB  cosAsinB
= (3/5)(12/13)  (4/5)(5/13)
= 56/65 
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bobpursley,
change the Sin A in the first term, and it works.

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Jonathan,
i knew that was wrong, coz my answer should be in the 50's haha..thanks though, great help
can i ask how you got sinA and sinB with that information :)
no point in me just copying it i would like to know how you got it 
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bobpursley,
In any angle, the sum of the squares of the cosines and sines is equal to one. Use that to solve.