Putem scrie;
a=21m
b=21n
a+b=21n+21m=21(m+n)=231
m+n=231:21=11
m=1, n=10....a=21 b=210
m=2 n=9....a=42 b=189
m=3,n=8....a=63 b=168
n=4,m=7 a=84 b=147
n=5,m=6 a=105 b=126
toate perechile (m, n) prime intre ele deci (a,b)=21 se respecta
celelate 5 solutii sunt simetrice