3m+4n-7=0
3a+4b+8=0
两式相减 m-a=4(b-n)/3+5
(m-a)^2+(n-b)^2
=[4(b-n)/3+5]^2+(n-b)^2
=25/9(b-n)^2+40(b-n)/3+25
=25/9[b-n+12/5]^2+9
>=9
根号[(m-a)^2+(n-b)^2]>=3