ask about limit the result value

hello, i wanna ask about c#, im from indonesia and i try to make some program with visual studio c# 2010

My based program is calculations, and I want to ask is, how do I limit the result value has been calculated to amount to little, for example, cos80 = 0.1736 in the calculator, but when calculations with the program that I created cos80 = 0.173648117. how do I equate the calculated value contained in the same program as the result of the calculation into the calculator?

please answer my question :)
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thanks before