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For $\dfrac{d^2y}{dx^2}+3\dfrac{dy}{dx}+2y=6e^{x}+\sin x$, the particular integral is:
A$6e^{x}+\frac{1}{10}(-3\cos x+\sin x)$
B$e^{x}+\frac{1}{10}(-3\cos x+\sin x)$
C$e^{x}+\frac{1}{10}(3\cos x-\sin x)$
D$e^{x}+\frac{1}{10}(-3\cos x-\sin x)$
Answer & Solution
Correct answer: B. $e^{x}+\frac{1}{10}(-3\cos x+\sin x)$
1. Handle the two forcing terms separately.
2. For $6e^{x}$, try $y_p=Ae^{x}$. Then $A+3A+2A=6A=6$, so $A=1$, giving $e^{x}$.
3. For $\sin x$, try $y_p=p\cos x+q\sin x$ and substitute.
4. Matching coefficients yields $p=-\dfrac{3}{10}$ and $q=\dfrac{1}{10}$.
5. Adding the two parts gives $e^{x}+\dfrac{1}{10}(-3\cos x+\sin x)$. Keeping the coefficient 6 skips solving for $A$.
_Source: NECTA ACSEE 2023 Advanced Mathematics 142/2, Question 7: Differential Equations_
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