Posted by **MIK** on Thursday, May 17, 2012 at 4:43pm.

Given sin a = 15/17 with a in Quadrant I and cos beta = 4/5 with beta in QuandrantIV, find the exact value of cos(a+beta)

- trig -
**Reiny**, Thursday, May 17, 2012 at 6:19pm
sin a = 15/17 , with a in I, so cos a = 8/17

cos b = 4/5, with b in IV, then sin b = -3/5

cos(a+b) = cosa cosb - sina sinb

= (8/17)(4/5) - (15/17)(-3/5)

= (32 + 45)/85

= 77/85

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