I was reading the uniform random variable from sheldon ross
it is given that X is a uniform random variable on the interval $(\alpha,\beta)$ if the probability density function of X is given by
$f(x)=\left\{ \frac{1}{\beta-\alpha}\, if \, \alpha \lt x\lt \beta \\ 0\, otherwise\right\}$
Since F(a)=$\int_{- \infty}^{a}f(x)dx$, it follows from above that the distribution function of a uniform random variable on the interval
$(\alpha,\beta)$ is given by
$F(a)=\left\{ 0 \, a \leq \alpha \\ \frac{a-\alpha}{\beta-\alpha} \; \alpha \lt a\lt \beta\\1 \,\, a \geq \beta\right\}$
My query is how they derived the distribution function from the density function?