a   c   a·d   b·c   a·d + b·c
  — + — = ——— + ——— = —————————
  b   d   b·d   b·d      b·d


  a   c   a·d   b·c   a·d - b·c
  — - — = ——— - ——— = —————————
  b   d   b·d   b·d      b·d


  a   c   a·c
  — · — = ———
  b   d   b·d


   a    a
   —    —·b
   b    b       a     a·d    a·d
  —— = ———— = ——— = ————— = ———
   c    c      c·b   c·b     c·b
   —    —·b    ———   ———·d          ovvero:
   d    d       d     d

            -1
       a  c    a d   a·d
   … = —·(—) = —·— = ———
       b  d    b c   b·c