Rationale Funktion/Quotient aus linearen Polynomen/Keine Extrema/Aufgabe/en

Prove that a non-costant rational function of the shape

$f(x)={\frac {ax+b}{cx+d}}$

(with ${}a,b,c,d\in \mathbb {R} ,a,c\neq 0$), has no local extrema.