Question 194740
(a+c)/(b+d) = (a-c) / (b-d)
cross  multiply
ab -ad +bc -cd  = ab +ad -bc -cd
cancelling  ab  and  -cd,  on  both  sides  of  eqn
combining  like  terms
-2ad = -2bc
dividing  both  sides  by (-2)
ad =bc
dividing  both  sides  by (bd)
a/b = c/d,,,,,,,,,as  required