Asked by Jonathan

if cosA = 4/5; sin <0; cosB = 12/13; 0<B<90, determine sin(A-B)

thanks ... :)
Answered by bobpursley
The first pairing says that sinA is -4/5

If cosB is 12/13, then sinB is 7/13

Sin(A-B)=CosASinB-SinACosB, right?
Answered by Jonathan
yeah that's right, can u go on?
Answered by bobpursley
I could, but I wont. However, I will be happy to critique your work or thinking
Answered by bobpursley

Error: sinA=-3/5

in all cases, sin^2 Theta + cos^2 Theta=1
Answered by Jonathan
well, i got:

(-4/5)(12/13) - (4/5)(7/13)
therefore

(-48/65) - (28/65)

therefore:

answer = (-76/65)
Answered by 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(A-B) = sinAcosB - cosAsinB
= (-3/5)(12/13) - (4/5)(5/13)
= -56/65
Answered by bobpursley
change the Sin A in the first term, and it works.
Answered by 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
Answered by bobpursley
In any angle, the sum of the squares of the cosines and sines is equal to one. Use that to solve.
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