A:2400π﹣1800 3
B:2400π﹣900 3
C:1200π﹣900 3
D:π﹣1800 3
200175 =____ (填小数)(保留三位小数)
a+2+ a22−a =2+a+ a22−a …第一步
=(2+a)(2﹣a)+a2…第二步
=2﹣a2+a2…第三步
=2…第四步
I've asked him __________ us for dinner, but he said he was quite busy.