std.math module

[greatsam4sure via Digitalmars-d-learn]

import std.math;
import std.stdio;

cos(90*PI/180) = -2.7e-20 instead of zero. I will appreciate any 
help. thanks in advance.

tan(90*PI/180) = -3.689e+19 instead of infinity. What is the best 
way to use this module