y=x^4/(x-1)
=(x^4-1+1)/(x-1)
=(x+1)(x^2+1) +1/(x-1)
=x^3 +x^2+ x+1 +1/(x-1)
所以
y'=3x^2 +2x +1 -1/(x-1)^2
y"=6x +2 +2/(x-1)^3
y"'=6 - 6/(x-1)^4
……
y(n)= n! *(-1)^n / (x-1)^(n+1),n大于等于4