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Question 84 of 105

Q.Let X be a continuous random variable and f(x)f(x) is defined as :
[!FORMULA] f(x)={kx(1−x)10,0<x<10,otherwisef(x)=\begin{cases}kx(1-x)^{10}, & 0<x<1\\0, & \text{otherwise}\end{cases}
find the value of k.

Puducherry TnboardTamil Nadu HSC (DGE) Board 2020Subjective· 3mImportance★★★★★
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Uses the normalisation condition ∫−∞∞f(x) dx=1\int_{-\infty}^{\infty}f(x)\,dx=1 together with the Beta-function integral to solve for kk.

  1. Since ff is a probability density function, the total area under it must equal 11: ∫01kx(1−x)10 dx=1\displaystyle\int_0^1 kx(1-x)^{10}\,dx=1 (as f(x)=0f(x)=0 outside (0,1)(0,1)).
  2. Pull the constant out: k∫01x(1−x)10 dx=1k\displaystyle\int_0^1 x(1-x)^{10}\,dx=1.
  3. Evaluate ∫01x(1−x)10 dx\displaystyle\int_0^1 x(1-x)^{10}\,dx using the substitution u=1−xu=1-x (so x=1−ux=1-u, dx=−dudx=-du; limits flip from x:0→1x:0\to1 to u:1→0u:1\to0): ∫01(1−u)u10 du=∫01(u10−u11)du\displaystyle\int_0^1(1-u)u^{10}\,du=\int_0^1\left(u^{10}-u^{11}\right)du. …

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