Re: Tough problem...

If you pass a large argument to sin or cos, the
result is not accurate.

As others explicited, this is to be expected...

The solution is not to pass a large argument in the first place, which can often be achieved if you're dealing with a "physical" problem (in opposition to "mathematical" problem). The "how" will depend on the problem, and can range from separating values (number of rotations + angle instead of an absolute angle) to transforming the math (using trigonometric formulas, quaternions, etc.), using a converging series or introducing a well chosen approximation.

Eric
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