Posted by **donny** on Friday, April 30, 2010 at 9:08pm.

can someone please help me with this problem. i'm doing my review right now and i'm stuck?

Find cos(s + t) given that cos s = 1/3, with s in quadrant 1, and sin t = -1/2, with t in quadrant 4.

- trig -
**Reiny**, Friday, April 30, 2010 at 9:20pm
s in I

if cos s = 1/3, then sin s = √8/3

t in IV

if sin t = -1/2, then cos t = √3/2

cos(s+t)

= cosscost - sinssint

= (1/3)(√3/2) - (√8/3)(-1/2)

=(√3 + √8)/6

- trig -
**donny**, Friday, April 30, 2010 at 9:49pm
this is the wrong answer..

- trig -
**Reiny**, Friday, April 30, 2010 at 11:02pm
I am 100% sure my answer is right.

What have they got?

Is is (√3 + 2√2)/6 perhaps.

If it is, I will let you think about it.

- trig -
**donny**, Friday, April 30, 2010 at 11:27pm
yes, (√3 + 2√2)/6 is the correct answer/form. thank you for your help!

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