a)a/b=c/d => ad=bc
15+bc/ad = 15+1 =16 (deoarece bc=ad de aceea bc/ad=1)
b)
a/b=c/d=2 => a/b=2 => a=2b ;
c/d=2 => c=2d si a•b•c•d=900
2b•b•2d•d=900 => 4(bd)²=900 => (bd)²=900:4 => (bd)²=225 => bd=15
.Acum aflam pe ac: a•b•c•d=900 |•4 <=> a•2b•c•2d=3600 <=> a•a•c•c=3600 <=> (ac)²=3600 => ac=60