The random variable X can take only values 0, 1, 2. Given that $P(X=0)=P(X=1)=ρ$ and $E(X^2)=E(X)+1,$ then the value of ρ is :
Answer & explanation
Correct answer: option 1
The correct answer is option (1) → $\frac{1}{4}$
$P(X=0)=P(X=1)=ρ$ [Given]
let $P(X=2)=ρ_2$, since the total probability must be 1,
$ρ+ρ+ρ_2=1$
$⇒ρ_2=1-2ρ$
and,
$E(X^2)=E(X)+1$ [Given]
$E(X)=0.P(X=0)+1.P(X=1)+2.P(X=2)$
$=0.ρ+1.ρ+2(1-2ρ)$
$=2-3ρ$
$E(X^2)=0^2.P(X=0)+1^2.P(X=1)+2^2.P(X=2)$
$=0.ρ+1.ρ+4(1-2ρ)$
$=4-7ρ$
$∴4-7ρ=(2-3ρ)+1$
$⇒1=4ρ$
$⇒ρ=\frac{1}{4}$