Gradient descent for SLE

            def cal_grad(XX,dx):
                Gstore=[]
                x1,x2,x3=XX[0],XX[1],XX[2]
                y1=8*(x1)-1*(x2)-8*(x3)
                y11=8*(x1+dx)-1*(x2)-8*(x3)
                g1=(y11-y1)/dx
                x1-=dx
                Gstore.append(g1)
                y2=-4*x1+2*x2+9*x3
                y22=-4*(x1)+2*(x2+dx)+9*(x3)
                g2=(y22-y2)/dx
                x2-=dx
                Gstore.append(g2)
                y3=-8*x1-9*x2+2*x3
                y33=-8*(x1)-9*(x2)+2*(x3+dx)
                g3=(y33-y3)/dx
                x3-=dx
                Gstore.append(g3)
                return Gstore

            X=[0,0,0]
            step=0.01
            iteri=0
            while(True):
                x1,x2,x3=X[0],X[1],X[2]
                f1=8*x1-1*x2-8*x3-9
                f2=-4*x1+2*x2+9*x3+5
                f3=-8*x1-9*x2+2*x3+7
                f=f1*f1+f2*f2+f3*f3
                print(x1,x2,x3)
                Y=f
                if (Y<0.0001):
                    break
                dx=step/10
                G=cal_grad(X,dx)
                Xnew=X               
                for i in range(0,3):        
                    Xnew[i]-=G[i]*step
                x11,x22,x33=Xnew[0],Xnew[1],Xnew[2]
                f11=8*x11-1*x22-8*x33-9
                f22=-4*x11+2*x22+9*x33+5
                f33=-8*x11-9*x22+2*x33+7
                fnew=f11*f11+f22*f22+f33*f33
                Ynew=fnew
                if (Ynew<Y):          
                    X=Xnew           
                    step=min(0.1,step*1.25)
                else:                  
                    step=step/1.25
                iteri+=1
            print(iteri)

here after first step Ynew>Y.so X value should be taking the previous value.but somehow X is taking the new X value(Xnew here in my code).Why? plz help

I checked but it's not working.Is my code wrong?

yes you can suggest me