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Calculus

GATE CSE 2014 Set 3 · Q47

MCQ2 marksEasy

Time

The value of the integral given below is ∫0πx2cos⁡x dx\int_0^\pi x^2 \cos x \, dx

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Solution

Step-by-step verified solution

Official answer: A) -2

Calculus

To evaluate the definite integral:

I=∫0πx2cos⁡x dxI = \int_0^\pi x^2 \cos x \, dx

Use integration by parts:

∫u v′ dx=uv−∫u′v dx\int u \, v^{\prime} \, dx = u v - \int u^{\prime} v \, dx

Step 1: Let u=x2u = x^2 and v′=cos⁡x  ⟹  u′=2xv^{\prime} = \cos x \implies u^{\prime} = 2x and v=sin⁡xv = \sin x.

I=[x2sin⁡x]0π−∫0π2xsin⁡x dxI = \left[ x^2 \sin x \right]_0^\pi - \int_0^\pi 2x \sin x \, dx

Since sin⁡π=0\sin \pi = 0 and sin⁡0=0\sin 0 = 0, the boundary term is:

[x2sin⁡x]0π=π2(0)−0(0)=0\left[ x^2 \sin x \right]_0^\pi = \pi^2(0) - 0(0) = 0

Thus,

I=−2∫0πxsin⁡x dxI = -2 \int_0^\pi x \sin x \, dx

Step 2: Integrate ∫0πxsin⁡x dx\int_0^\pi x \sin x \, dx by parts again with u=xu = x and v′=sin⁡x  ⟹  u′=1v^{\prime} = \sin x \implies u^{\prime} = 1 and v=−cos⁡xv = -\cos x:

∫0πxsin⁡x dx=[−xcos⁡x]0π−∫0π(−cos⁡x) dx\int_0^\pi x \sin x \, dx = \left[ -x \cos x \right]_0^\pi - \int_0^\pi (-\cos x) \, dx
=(−πcos⁡π−(−0))+[sin⁡x]0π= \left( -\pi \cos \pi - (-0) \right) + \left[ \sin x \right]_0^\pi
=−π(−1)+(sin⁡π−sin⁡0)=π+0=π= -\pi(-1) + (\sin \pi - \sin 0) = \pi + 0 = \pi

Step 3: Substitute back into II:

I=−2(π)=−2πI = -2(\pi) = -2\pi

Hence, the correct option is (A).

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This Calculus previous year question is Q47 of the GATE CSE 2014 Set 3 paper. Find more GATE CSE PYQs on Calculus in Engineering Mathematics, or browse all GATE CSE PYQs.

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Calculus in GATE CSE

Calculus has 40 GATE CSE PYQs across 25 papers from 2011 to 2026. 8 of them are 2-mark MCQs like this one, and GATE CSE 2014 Set 3 has 3.

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GateAI solution cardGATE CSE 2014 Set 3 · Q47 · MCQ

The value of the integral given below is 0 x2 cos x dx

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