∵a-b=5∴b=a-5把b=a-5代入b-c=2中，得：a-5-c=2即：A-C=7
a-b=5,a=5+b,b-c=2,c=b-2,a-c=5+b-b+2=7
(a-b)+(b-c)=a-b+b-c=a-c=5+2=7
A-C=a-b-（c-b）=a-b+（b-c）=5+2=7
