通常，你只会在这两种情况下用替代法，或者这可以极大地简化被积函数，或者就是这可以简化，积分的上下限。

I mean, normally, you would only do this kind of substitution if either it simplifies a lot the function you are integrating, or it simplifies a lot the region on which you are integrating.

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这极大地限制了阶乘函数的可能范围。

This limits greatly the possible range of your factorial function.

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函数和方法的默认参数极大地减少了需要创建的重载方法数量。

Default arguments on functions and methods greatly reduce the number of overloaded methods you need to create.

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