a)a/2=b/5=c/7=k;
a=2k;
b=5k;
c=7k;
ac-ba=2k•7k-5k•2k=2k(7k-5k)=2k•2k=4k^2=(2k)^2=>ac-ba este patrat perfect;
ac+bc=c(a+b)=7k(2k+5k)=7k•7k=49k^2=(7k)^2=>ac+bc este patrat perfect;
b)bc+ac=156+ab;
c(a+b)=156+ab;
7k(2k+5k)=156+2k•5k;
49k^2=156+10k^2;
49k^2-10k^2=156;
39k^2=156;
k^2=4;
k=2;
a=4;
b=10;
c=14
