原式=（3a）2-（b）2
=9a-b．
故答案为9a-b．
Tens of thousands of people have visited Expo 2010 Shanghai China ________ it opened in April. [ ]
A. for
B. before
C. since
D. while
涂膜的干燥的程度有以下（）种。
A、指干燥
B、半硬干燥
C、自动干燥
D、完全干燥
