Charlie.

this week we reviwed old stuff and took exams.
on friday we started chapter 10 section 1.
we can't use a calculater int his chapter.
first you hae to use the formluas:
cos(alpha+/- beta) = cos(alpha) cos(beta) -/+ sin(alpha) cos(beta)
& sin(alpha +/- beta) = sin(alpha) cos(beta) +/- cos(alpha) sin(beta)
alpha and beta come from the trig. chart and add or subtract to get the angle you are looking for.

one thing is where the problem says "suppose that sin or cos =", in these problems you have to figure out the othe cosines or sines yourself.
For Example:
suppose that sin (alpha) = 4/5 and sin (beta) = 5/13 where 9cosalpha cosbeta - sinalpha sinbeta
cosalpha cosbeta - (4/5)(5/13)
then to find out cosalpha and cosbeta 3/5 and cos beta = -12/13, soo..
(3/5)(-12/13) - (4/5)(5/13)
(-36/65) - (20/65)
= -56/65