s :=t=t(x)->(a*y(x1)*(1-t)^3+((2*a+b)*y(x1)+a*d1*h1)*(1-t)^2*t+((a+2*b)*y(x2)-b*d2*h1)*(1-t)*t^2+b*y(x2)*t^3)/(a*(1-t)+b*t) ;
t = (x-x1)/(x2-x1);
h1 = x2-x1;
怎么对x一阶和二阶求导