trig
posted by donny .
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.

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 
this is the wrong answer..

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. 
yes, (√3 + 2√2)/6 is the correct answer/form. thank you for your help!