Posted by **Anonymous** on Wednesday, November 12, 2008 at 7:57pm.

Assume that A and B are in standard position and that sin A = 1/2, cos A >0, tan B = 3/4, and sinB<0

Cos (A-B)

- trig - identies -
**Reiny**, Wednesday, November 12, 2008 at 8:42pm
from sin A = 1/2, cos A >0, we know that A has to be in quadrant I, so cosA = √3/2

from tan B = 3/4, and sinB<0, we know that B must be in quadrante III, so sinB=-3/5 and cosB = -4/5

Cos(A-B) = cosAcosB + sinAsinB

= √3/2(-4/5) + 1/2(-3/5)

= (-4√3 - 3)/10

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