Posted by House on .
If a and B are two angles in Quadrant 2 such that tan a=1/2 and tan B= 2/3, find cos(a+b)?
tan(a) = 1/2
oppsite side = 1: adjacentside = 2
hypotenuse = sqrt(1+4) = sqrt(5)
sin(a) = 1/ã5
cos(a) = 2/ã5
tan(b) = 2/3
opposite side = 2 and adjacentside = 3
hypotenuse = sqrt(4+9) = ã13
sin(b) = 2/ã13
cos(b) = 3/ã13
cos(a+b) = cosa cosb  sina sinb
=(2/ã5)(3/ã13)  (1/ã5)(2/ã13)
= 6/ã65  2/ã65
= 4/ã65
right?

Math 
bobpursley,
I didn't check all, but you have the technique. You can easily check these on your calculator.