y= 1/(x2+6x+8) =（1/(x+2)-1/(x+4)）/2一阶导 ：(-1)*1!*(1/(x+2)^2-1/(x+4)^2)/2二阶导：(-1)^2*2!*(1/(x+2)^3-1/(x+4)^3)/2.n阶导：(-1)^n*n!*(1/(x+2)^(n+1)-1/(x+4)^(n+1))/2用递归证明