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Exercise 11.13 · Q20

Q.∫e−7xsin⁡5x dx\displaystyle\int e^{-7x}\sin 5x\,dx is

(1) e−7x74[−7sin⁡5x−5cos⁡5x]+c\dfrac{e^{-7x}}{74}[-7\sin5x-5\cos5x]+c
(2) e−7x74[7sin⁡5x+5cos⁡5x]+c\dfrac{e^{-7x}}{74}[7\sin5x+5\cos5x]+c
(3) e−7x74[7sin⁡5x−5cos⁡5x]+c\dfrac{e^{-7x}}{74}[7\sin5x-5\cos5x]+c
(4) e−7x74[−7sin⁡5x+5cos⁡5x]+c\dfrac{e^{-7x}}{74}[-7\sin5x+5\cos5x]+c
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Apply the standard eaxsin⁡bxe^{ax}\sin bx Bernoulli result with a=−7a=-7.

Step 1. Use ∫eaxsin⁡bx dx=eaxa2+b2[asin⁡bx−bcos⁡bx]+c\displaystyle\int e^{ax}\sin bx\,dx=\dfrac{e^{ax}}{a^2+b^2}[a\sin bx-b\cos bx]+c with a=−7a=-7, b=5b=5. …

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