If a : b = c : d, the according to dividendo property
a + b : b = c + d : d
a - b : a = c - d : c
a - b : b = c - d : d
a + b : a = c + d : c
Let ABCD be a quadrilateral with diagonals AC and BD. Prove the following statements (Compare these with the previous problem);
(a) AB + BC + CD > AD;
(b) AB + BC + CD + DA > 2AC;
(c) AB + BC + CD + DA > 2BD;
(d) AB + BC + CD + DA > AC + BD.