# Plotting the tangent line for newton raphson method

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Patrick Ofenloch on 3 Dec 2021
How can I plot the tangent lines for the newton raphson method?
I tried it this way... (have a closer look at line 33 - 38)
% Nullstellen von
%
% y(x) = x^2 - 2
%
clear, clc
fx = @(x) x.^2 -2
dfdx = @(x) 2*x
n_step = 7 ; % Anzahl der durchzuführenden Schritte für Newtonverfahren
x0 = 5 ; % Startwert
%%%% newton-verfahren %%%%%%%%%%%%
x = zeros(n_step+1,1) ;
x(1) = x0 ;
for i=1:n_step
x(i+1) = - fx(x(i)) / dfdx(x(i)) + x(i) ;
end
y = fx(x) ; % zugehörige y-werte / Daten zum Plotten
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
xx = [1 : .01 : 5] ; % Daten zum plotten blaue Linie
yy = fx(xx) ; % Daten zum plotten blaue Linie
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
for j=1:3
yt(j+1) = dfdx(x(j)) * (x(j)-x(j))+fx(x(j))
xt(j+1) = -fx(x(j))/dfdx(x(j)) +x(j)
end
figure(1), clf
subplot(2,1,1), grid on, hold on
plot(xx,yy)
plot(x,y,'r--*')
title('Newton-Raphson-Verfahren')
legend('y=x^2-2','Newton-Verfahren','location','northwest')
subplot(2,1,2), grid on, hold on
plot(xx,yy)
plot(x,y,'r*')
plot(xt,yt,'r.-')
title('Newton-Raphson-Verfahren')
legend('y=x^2-2','Newton-Verfahren','Tangenten','location','northwest')
%%%%% bildschirmausgabe *****************
format long
disp('************ Newton-Verfahren ***************')
disp(' x y=f(x) ')
disp([ x y ])
format short
The tangents should be generated in the secon for slope. (line 33 - 38)
After that, i want to plot it in the second subplot.
exception:
Thanks for helping me.

Alan Stevens on 3 Dec 2021
Like this?
fx = @(x) x.^2 -2;
dfdx = @(x) 2*x;
n_step = 7 ; % Anzahl der durchzuführenden Schritte für Newtonverfahren
x0 = 5 ; % Startwert
%%%% newton-verfahren %%%%%%%%%%%%
x = zeros(n_step+1,1) ;
x(1) = x0;
for i=1:n_step
x(i+1) = - fx(x(i)) / dfdx(x(i)) + x(i) ;
end
y = fx(x) ; % zugehörige y-werte / Daten zum Plotten
% Tangent line calcuations
xt = zeros(2*n_step+1,1);
yt = zeros(2*n_step+1,1);
xt(1) = x(1); yt(1) = y(1);
for i = 1:n_step
xt(2*i) = x(i+1);
xt(2*i+1) = x(i+1);
yt(2*i) = 0;
yt(2*i+1) = fx(x(i+1));
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
xx = 1 : .01 : 5 ; % Daten zum plotten blaue Linie
yy = fx(xx) ; % Daten zum plotten blaue Linie
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
figure(1), clf
subplot(2,1,1), grid on, hold on
plot(xx,yy)
plot(x,y,'r--*')
title('Newton-Raphson-Verfahren')
legend('y=x^2-2','Newton-Verfahren','location','northwest')
subplot(2,1,2), grid on, hold on
plot(xx,yy)
plot(x,y,'r*')
plot(xt,yt,'r.-')
title('Newton-Raphson-Verfahren')
legend('y=x^2-2','Newton-Verfahren','Tangenten','location','northwest')
%%%%% bildschirmausgabe *****************
format long
disp('************ Newton-Verfahren ***************')
************ Newton-Verfahren ***************
disp(' x y=f(x) ')
x y=f(x)
disp([ x y ])
5.000000000000000 23.000000000000000 2.700000000000000 5.290000000000001 1.720370370370370 0.959674211248285 1.441455368177650 0.077793578448165 1.414470981367771 0.000728157131505 1.414213585796884 0.000000066252480 1.414213562373095 0.000000000000000 1.414213562373095 -0.000000000000000
format short
Patrick Ofenloch on 3 Dec 2021
perfect,
thank you!