Question 823644
 Let sin(s) = (-1/4), with s in quadrant 4 
and cos(t) = (-4/5), with t in quadrant 2.
-----
Find cos(s)::
Since sin(s) = y/r = -1/4, y = -1 and r = 4
Therefore x = sqrt[4^2-1^2] = sqrt(15)
And cos(s) = x/r = sqrt(15)/4
-------------------------------------------
Find sin(t)::
Since cos(t) = x/r = -4/5, x = -4 and r = 5
Therefore y = sqrt[5^2-4^2] = 3
So sin(t) = y/r = 3/5
--------------------------------
Find sin(s+t) = sin(s)cos(t)+cos(s)sin(t)
-----
= (-1/4)(-4/5)+(sqrt(15)/4)(3/5)
------
= (1/5)+(3sqrt(15)/20)
-----
= [4+3sqrt(15)]/20
-------------
= 0.7809
==============
Cheers,
Stan H.
==============