n+1又n-1分之1=n*1又n-1分之1
就是左边=n+1+1/(n-1)=[(n+1)(n-1)+1]/(n-1)=n^2/(n-1)
右边=n*(1+1/(n-1))=n*[(n-1+1)/(n-1)]=n^2/(n-1)=左边