16) Let f be a function defined on [a, b] such that $f (x) > 0$, for all$x \: \: \epsilon \: \: ( a,b)$. Then prove that f is an increasing function on (a, b).

$f(x)= x^3 > 0 , (a.b)$
$f{'}(x)= 3x^2 > 0 , (a,b)$