x分之x-y除以括号x-x分之2xy-y的平方=(x-y/x) / (x-(2xy-y^2/x))=(x-y/x) / (x^2-2xy+y^2)/x=(x-y/x) * x/(x-y)^2=1/(x-y)=1/(2009-2010)=-1
6
