1. ## [SOLVED] cos(a+b)

if sin (a) =40/41, and a lies in quadrant 1
and sin b = 40/41 and lies in quadrant II
what is the exact value of cos(a+b)?

2. Originally Posted by So0pahBoy
if sin (a) =40/41, and a lies in quadrant 1
and sin b = 40/41 and lies in quadrant II
what is the exact value of cos(a+b)?
HI

$\cos a =\frac{9}{41}$

$\cos b=-\frac{9}{41}$

Now $\cos (a+b)=\cos a\cos b-\sin a\sin b$

Substitute appropriately .

3. Thanks, but, how did you find the cos?

4. Originally Posted by So0pahBoy
Thanks, but, how did you find the cos?
Draw 2 triangles in the 1st and 2nd quadrant . Read the qusetion again to know which belongs to which . Then phythagoras would give the remaining side . In the 1st quadrant , everything is positive but in the 2nd , only sin is positive , cos is negative .

5. ahh, okay, thanks!