Math functions

[Jarmo]

Hello,

I hope this is a right place to ask some questions about C - functions. 
I have created an application called MathPlanner and there is problems
with math functions and I don't know what is wrong. These problems
does appear in other applications as well.

When I'am trying to calculate cos(90) the answer should be 0 but it is
something like 1.234e-20.  Many other functions will also give same
kind of answers. So. How should I deal with this ?
Is there any information available in this topic ?

Best regards,
Jarmo