A, either take the integral and then the derivative, or take the derivative and then the integral.

? Or derivative definite integral is used in combination for many times;

B, using the summation formula of infinite proportional sequence whose common ratio is less than 1.

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It should be pointed out that:

When using definite integral, we should pay special attention to the lower limit of integral, otherwise,

There will be a mistake.

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The following five pictures are examples for the landlord's reference.

If you click to enlarge, the picture will be clearer.

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