Cosines

I'm trying to work out an angle of a non-right angel triangle. I have the 3 sides and the equation works (checked with excel and a cad package and on calculator) but the angle always comes out wrong in Qbasic. The number is correct until I try to make an angle of it and then it's at least 1% out?
at this point it's correct
as2 = (((ls * ls) + (kpl * kpl) - (tl * tl)) / (2 * (kpl * ls)))
however I go from here to convert it to an angle goes wrong.
if I work out a series of these and subtract them, the differences are correct but none of the actual position are?

Help