(3/2a的立方-3/4a+5/6)-(8/15a的立方-1/10a的平方-2/9)
=3/2a的立方-3/4a+5/6-8/15a的立方+1/10a的平方+2/9
=29/30a的立方+1/10a的平方-3/4a+19/18