Question

Establish for which constants c the following function is a density.

Answer 1

By "density" I assume you mean "probability density function". For this to be the case for $f(x)$, we require

$\displaystyle\int_{-\infty}^\infty f(x)\,\mathrm dx=1$

Since

$f(x)=\begin{cases}cx^{1/2}&\text{for }0$

you have

$\displaystyle\int_{-\infty}^\infty f(x)\,\mathrm dx=\int_0^2cx^{1/2}\,\mathrm dx=\dfrac{2c}3x^{3/2}\bigg|_{x=0}^{x=2}=\dfrac{2^{5/2}c}3=1$

which means

$c=\dfrac3{2^{5/2}}=\dfrac3{4\sqrt2}$