//    PG (140)

X = [2.0,2.1,2.2,2.3,2.4]
X1 = X(1,1)
X2 = X(1,2)
X3 = X(1,3)
X4 = X(1,4)
X5 = X(1,5)
deff('[y]=f(x)','y=sqrt(x)')
Y = [f(X1) f(X2) f(X3) f(X4) f(X5)]
Y1 = Y(1,1)
Y2 = Y(1,2)
Y3 = Y(1,3)
Y4 = Y(1,4)
Y5 = Y(1,5)

//    Difference

//    f[X1,X2]
(f(X2) - f(X1))*10
//    f[X2,X3]
(f(X3) - f(X2))*10
//    f[X3,X4]
(f(X4) - f(X3))*10
//    f[X4,X5]
(f(X5) - f(X4))*10

//    D^2 * f[Xi]

((f(X3)-f(X2)) - (f(X2)-f(X1))) * 50
((f(X4)-f(X3)) - (f(X3)-f(X2))) * 50
((f(X5)-f(X4)) - (f(X4)-f(X3))) * 50