Pdf of area and circumference of a circle

[uva123]

Homework Statement

Suppose that the radius X of a circle is a random variable having the following p.d.f.:
f(x)={ (1/8)(3x=1) for 0<x<2
0 otherwise
Determine the p.d.f. of the area of the circle and the circumference of the circle.

Homework Equations

Area=$$\Pi$$r²
Circumference=2$$\Pi$$r

The Attempt at a Solution

can i just insert f(x) in for r in both equations to generate the pdf??
meaning...
if g is the area of the circle with r=f(x)
g(f(x))={$$\Pi$$[(1/8)(3x+1)]² for 0<x<2
0 otherwise
likewise for circumference

 

[lanedance]

no, they will be related by
$$|f(r)dr| = |f(c)dc|$$

where c is circumference & the change dc correspond to the incremental dr