JEE Advance - Mathematics (2024 - Paper 1 Online - No. 13)

Let $X$ be a random variable, and let $P(X=x)$ denote the probability that $X$ takes the value $x$. Suppose that the points $(x, P(X=x)), x=0,1,2,3,4$, lie on a fixed straight line in the $x y$-plane, and $P(X=x)=0$ for all $x \in \mathbb{R}-\{0,1,2,3,4\}$. If the mean of $X$ is $\frac{5}{2}$, and the variance of $X$ is $\alpha$, then the value of $24 \alpha$ is _____________.
Answer
42

Explanation

Let equation of line is $$\mathrm{y=mx+c}$$

$$\mathrm{x}$$ 0 1 2 3 4 $$\mathrm{R-\{0,1,2,3,4\}}$$
$$\mathrm{P(x)}$$ $$\mathrm{c}$$ $$\mathrm{m+c}$$ $$\mathrm{2m+c}$$ $$\mathrm{3m+c}$$ $$\mathrm{4m+c}$$ 0

$$\sum_{x=0}^4(m x+c)=1 \Rightarrow 10 m+5 c=1 \Rightarrow 2 m+c=\frac{1}{5}\quad \text{.... (i)}$$

$$\begin{aligned} & \text { mean }=\sum x_i P_i=\sum_{i=0}^4\left(m x_i+c\right) \cdot x_i=30 m+10 c=\frac{5}{2} \\ & \therefore 3 m+c=\frac{1}{4} \ldots(2) \\ & \text { from (1) and (2) } \mathrm{m}=\frac{1}{20}, \mathrm{c}=\frac{1}{10} \\ & \Sigma \mathrm{P}_{\mathrm{i}} \mathrm{x}_{\mathrm{i}}^2=\sum_{\mathrm{i}=0}^4\left(\mathrm{mx}_{\mathrm{i}}+\mathrm{c}\right) \mathrm{x}_1^2 \\ & =\sum_{\mathrm{i}=0}^4\left(\mathrm{mx}_{\mathrm{i}}^3+c \mathrm{x}_{\mathrm{i}}^2\right) \Rightarrow 100 \mathrm{~m}+30 \mathrm{c}(\text { Now putting } \mathrm{m} \text { and } \mathrm{c}) \\ & \Rightarrow \Sigma \mathrm{P}_{\mathrm{i}}^2=5+3=8 \\ & \text { Variance }=\Sigma \mathrm{P}_{\mathrm{i}} \mathrm{x}_{\mathrm{i}}^2-\left(\Sigma \mathrm{P}_{\mathrm{i}} \mathrm{x}_{\mathrm{i}}\right)^2=8-\left(\frac{5}{2}\right)^2=\frac{7}{4} \\ & \therefore 24 \alpha=42 \end{aligned}$$

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