Urèete derivaci funkce:
   $$
      y=\frac{ax+b}{a+b}.
   $$
   \reseni
   $$
      y'(x) =\Biggl( \biggl( \frac1{a+b} \biggr) \cdot (ax+b)
            \Biggr)' = \frac{a}{a+b}
   $$
