b+d=5 hoac 15
a+c=15 hoac 14
b-d-1=5 hoac 15
a-c=1 hoac 2
tu do trien khai tiep
====the a giai chi tiet cach khac===
ab,0+cd,4=158,4
ab,0-cd,4=15,6
tong hai so =158,4
hieu hai so=15,6
2.ab=158,4+15,6=174,0
ab=87,0
cd,4=87,0-15,6=71,4
DS: a=8; b=7;c=7;d=1