5a+4b 2
---------- = ---
5a+7b 3
==> 3*(5a+4b)=2*(5a+7b) ==> 15a+12b=10a+14b ==> 15a-10a=14b-12b
==> 5a=2b
b
------ ; amplificam cu 2 (inmultim si numaratorul si numitorul cu 2) ==>
a+b
2b 5a 5a 5a 5
---------- = ----------- = ----------- = -------- = ----
2(a+b) 2a+2b 2a+5a 7a 7