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MTH 102 MTH 102 Past Questions Objective Question

Question

Simplify ∫ ex sin x dx.
Options
A <i>(e<sup>x</sup>/4)(cos x - sin<sup>2</sup> x) + C</i>
B <i>(e<sup>x</sup>/3)(cos<sup>2</sup> x + sin x) + C</i>
C <i>(e<sup>x</sup>/2)(sin x - cos x) + C</i>
Correct Answer
D <i>e<sup>x</sup>(cos x - sin x) + C</i>
Correct Answer

Option C is the correct answer.

Detailed Explanation

This integral requires integration by parts twice. The formula for integration by parts is ∫ u dv = uv - ∫ v du.Let I = ∫ ex sin x dx.First application of integration by parts:Let u = sin x and dv = ex dx.Then du = cos x dx and v = ex.I = ex sin x - ∫ ex cos x dx (Equation 1)Second application of integration by parts (on ∫ ex cos x dx):Let u = cos x and dv = ex dx.Then du = -sin x dx and v = ex.∫ ex cos x dx = ex cos x - ∫ ex (-sin x) dx∫ ex cos x dx = ex cos x + ∫ ex sin x dx (Equation 2)Substitute Equation 2 back into Equation 1:I = ex sin x - (ex cos x + ∫ ex sin x dx)I = ex sin x - ex cos x - IAdd I to both sides:2I = ex sin x - ex cos xI = (ex/2)(sin x - cos x) + C.This matches option C.

Hint

This is a classic integration by parts problem that requires two applications of the technique. Be careful with the signs and remember to substitute the integral back into the original equation.

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