Independent solution

How to solve this Density Normalization question

Setup

Setup

Write the nonnegative density with an unknown normalizing constant and normalize it by substituting u=1+x².

f(x)=cx(1+x2)9,x0f(x)=c\frac{x}{(1+x^2)^9},\qquad x\ge0
0x(1+x2)9dx=116,c=16\int_0^\infty\frac{x}{(1+x^2)^9}\,dx=\frac1{16},\qquad c=16

Model

Model

Integrating the normalized density gives a convenient closed form for the distribution and survival functions.

F(t)=160tx(1+x2)9dx=1(1+t2)8F(t)=16\int_0^t\frac{x}{(1+x^2)^9}\,dx=1-(1+t^2)^{-8}
S(t)=(1+t2)8S(t)=(1+t^2)^{-8}

Compute

Compute

At the 2nd percentile, the survival probability is 0.98. Invert the survival function.

(1+t2)8=0.98(1+t^2)^{-8}=0.98
t=0.981/81=0.0502844883t=\sqrt{0.98^{-1/8}-1}=0.0502844883\ldots

Answer

Answer

The percentile rounds to 0.050.

0.050(D)\boxed{0.050\quad\text{(D)}}