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GATE 2016 EC (EC2) – Question 38

Engineering Mathematics · Probability and Statistics · 2 marks · Numerical answer

The joint density is $f_{X,Y}(x,y)=x+y$ for $0\le x\le1$ and $0\le y\le1$, and zero otherwise.
The probability $P(X+Y\le1)$ is ________.

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Correct answer: 0.32 to 0.34

Explanation

The event occupies the lower triangle of the unit square: $0\le x\le1$ and $0\le y\le1-x$. Integrate the joint density there:
$$P=\int_0^1\left[xy+\frac{y^2}{2}\right]_0^{1-x}dx.$$
The inner integral is $x(1-x)+(1-x)^2/2=(1-x^2)/2$. Its integral is $1/2-1/6=$ **1/3≈0.33333**, within 0.32–0.34.

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