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Answers for “cos x 的原函数是什么?”

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The final antiderivative is ex(sin⁡x+cos⁡x)2+C\frac{e^x(\sin x + \cos x)}{2} + C. This result is obtained by solving the algebraic equation formed when the original integral reappears after two rounds of integration by parts.

Conditions: The function integrated is excos⁡xe^x \cos x.; The constant of integration CC is included.; The algebraic solution for the recurring integral is correct.

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The general antiderivative of cos⁡x\cos x is sin⁡x+C\sin x + C, and the general antiderivative of exe^x is ex+Ce^x + C. In both cases, CC represents an arbitrary constant, indicating that any vertical shift of these base functions is also a valid antiderivative.

Conditions: The variable is xx.; The domain is all real numbers (or a connected interval thereof).

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Applying integration by parts twice returns the original integral ∫excos⁡xdx\int e^x \cos x dx on the right-hand side, but with a minus sign. This creates an algebraic equation where the unknown integral appears on both sides, allowing it to be solved by moving terms.

Conditions: The integrand is excos⁡xe^x \cos x.; Integration by parts is applied sequentially twice.; The cycle of derivatives for exe^x and cos⁡x\cos x repeats.