（a+3b-2c）（a-3b-2c）
原式=[（a-2c）+3b][（a-2c）-3b]
=（a-2c）2-（3b）2
=a2-4ac+4c2-9b2．
People try to avoid public transportation delays by using their own cars , and this _____ creates
further problems . [ ]
A. in short
B. in case
C. in doubt
D. in turn
可识别边缘的选取工具是（）
A.套索工具
B.多边形套索工具
C.磁性套索工具
D.魔术棒工具
