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Exercise 11.8 · Q2

Q.Integrate the following with respect to xx:

(i) e−3xsin⁡2xe^{-3x}\sin 2x
(ii) e−4xsin⁡2xe^{-4x}\sin 2x
(iii) e−3xcos⁡xe^{-3x}\cos x
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Each is Result 11.1 with a negative value of aa; substituting carefully (including the sign of aa) into the formula gives the answer directly.

Part (i): e−3xsin⁡2xe^{-3x}\sin2x. Here a=−3, b=2a=-3,\,b=2: ∫e−3xsin⁡2x dx=e−3x9+4[−3sin⁡2x−2cos⁡2x]+c=−e−3x13(3sin⁡2x+2cos⁡2x)+c\displaystyle\int e^{-3x}\sin2x\,dx=\dfrac{e^{-3x}}{9+4}\left[-3\sin2x-2\cos2x\right]+c=-\dfrac{e^{-3x}}{13}(3\sin2x+2\cos2x)+c.

Check: ddx[−e−3x13(3sin⁡2x+2cos⁡2x)]=3e−3x13(3sin⁡2x+2cos⁡2x)−e−3x13(6cos⁡2x−4sin⁡2x)=e−3x13[(9sin⁡2x+6cos⁡2x)−(6cos⁡2x−4sin⁡2x)]=e−3x13(13sin⁡2x)=e−3xsin⁡2x\dfrac d{dx}\left[-\dfrac{e^{-3x}}{13}(3\sin2x+2\cos2x)\right]=\dfrac{3e^{-3x}}{13}(3\sin2x+2\cos2x)-\dfrac{e^{-3x}}{13}(6\cos2x-4\sin2x)=\dfrac{e^{-3x}}{13}\left[(9\sin2x+6\cos2x)-(6\cos2x-4\sin2x)\right]=\dfrac{e^{-3x}}{13}(13\sin2x)=e^{-3x}\sin2x ✓.

Part (ii): e−4xsin⁡2xe^{-4x}\sin2x. Here a=−4, b=2a=-4,\,b=2: ∫e−4xsin⁡2x dx=e−4x16+4[−4sin⁡2x−2cos⁡2x]+c=−e−4x20(4sin⁡2x+2cos⁡2x)+c=−e−4x10(2sin⁡2x+cos⁡2x)+c\displaystyle\int e^{-4x}\sin2x\,dx=\dfrac{e^{-4x}}{16+4}\left[-4\sin2x-2\cos2x\right]+c=-\dfrac{e^{-4x}}{20}(4\sin2x+2\cos2x)+c=-\dfrac{e^{-4x}}{10}(2\sin2x+\cos2x)+c.

Check: ddx[−e−4x10(2sin⁡2x+cos⁡2x)]=4e−4x10(2sin⁡2x+cos⁡2x)−e−4x10(4cos⁡2x−2sin⁡2x)=e−4x10[(8sin⁡2x+4cos⁡2x)−(4cos⁡2x−2sin⁡2x)]=e−4x10(10sin⁡2x)=e−4xsin⁡2x\dfrac d{dx}\left[-\dfrac{e^{-4x}}{10}(2\sin2x+\cos2x)\right]=\dfrac{4e^{-4x}}{10}(2\sin2x+\cos2x)-\dfrac{e^{-4x}}{10}(4\cos2x-2\sin2x)=\dfrac{e^{-4x}}{10}\left[(8\sin2x+4\cos2x)-(4\cos2x-2\sin2x)\right]=\dfrac{e^{-4x}}{10}(10\sin2x)=e^{-4x}\sin2x ✓. …

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