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Official PYQ · 2023

GATE CS 2023 Q21 — Calculus: Limits, Continuity, Differentiability

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The value of the definite integral
∫−33∫−22∫−11(4x2y−z3)dz dy dx\int_{-3}^{3} \int_{-2}^{2} \int_{-1}^{1}\left(4 x^{2} y-z^{3}\right) \mathrm{d} z \mathrm{~d} y \mathrm{~d} x
is _________. (Rounded off to the nearest integer)
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Answer: 0

We can evaluate the given triple integral by splitting it into two parts using the linearity of integration:

I=∫−33∫−22∫−11(4x2y−z3) dz dy dx=I1−I2I = \int_{-3}^{3} \int_{-2}^{2} \int_{-1}^{1} (4 x^{2} y - z^{3}) \, \mathrm{d}z \, \mathrm{d}y \, \mathrm{d}x = I_1 - I_2

Since the limits of integration are independent constants, we can separate the variables for each part:

  1. First part (I1I_1): I1=(∫−334x2 dx)(∫−22y dy)(∫−111 dz)I_1 = \left( \int_{-3}^{3} 4x^2 \, \mathrm{d}x \right) \left( \int_{-2}^{2} y \, \mathrm{d}y \right) \left( \int_{-1}^{1} 1 \, \mathrm{d}z \right) Since yy is an odd function and the interval [−2,2][-2, 2] is symmetric about 00, we have: ∫−22y dy=0  ⟹  I1=0\int_{-2}^{2} y \, \mathrm{d}y = 0 \implies I_1 = 0
  2. Second part (I2I_2): I2=(∫−331 dx)(∫−221 dy)(∫−11z3 dz)I_2 = \left( \int_{-3}^{3} 1 \, \mathrm{d}x \right) \left( \int_{-2}^{2} 1 \, \mathrm{d}y \right) \left( \int_{-1}^{1} z^3 \, \mathrm{d}z \right) Similarly, since z3z^3 is an odd function and the interval [−1,1][-1, 1] is symmetric about 00, we have: ∫−11z3 dz=0  ⟹  I2=0\int_{-1}^{1} z^3 \, \mathrm{d}z = 0 \implies I_2 = 0

Combining both parts, we get:

I=I1−I2=0−0=0I = I_1 - I_2 = 0 - 0 = 0

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