The value of \(\displaystyle\int_0^1 \int_0^x (x^2 + y^2)dA\), where dA indicate small area in xy-plane, is

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UPSC ESE 2021 Official Paper 1
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  1. \(\frac{1}{2}\) sq. units
  2. \(\frac{1}{3}\) sq. units
  3. \(-\frac{1}{2}\) sq. units
  4. \(-\frac{1}{3}\) sq. units

Answer (Detailed Solution Below)

Option 2 : \(\frac{1}{3}\) sq. units
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Detailed Solution

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Concept:

\(\displaystyle\int_0^m \int_0^x f(x,y)dA\)

Since the inside limit is in terms of x, therefore we have to integrate first the 'y' terms and convert whole expression in terms of x.

Calculation:

Given:

\(\displaystyle\int_0^1 \int_0^x (x^2 + y^2)dA\)

\(\displaystyle\int_0^1\left[ \int_0^x (x^2 + y^2)dy\right]dx\;\)

\(\displaystyle\int_0^1\left( x^2[y]_0^x\;+\;\left[\frac{y^3}{3}\right]_0^x\right)dx\;\)

\(\displaystyle\int_0^1\left( x^3\;+\;\frac{x^3}{3}\right)dx\;\)

\(\displaystyle\int_0^1\left(\frac{4x^3}{3}\right)dx\;\)

\(\displaystyle \left(\frac{4x^4}{12}\right)_0^1\;\)

\(\displaystyle \frac{1}{3}\)

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