Newton Method

Derivation of the Method

If you want to find an value where a function, and the derivative can be calculated, Newton's Method is a good approach. This applet graphically steps through what is happening with the method. Step 1 : The function where we want to find the root. In this case it is solving for Step 2 : Solve for the derivative Step 3 : Choose an value to start. This is an art with Newton's method. Step 4 : Evaluate the function and the derivative at the value. Draw the point . Step 5 : The length of the segment shown is . Note: it may be negative so it is not really a length. Step 6 : Using the derivative, draw the tangent line. We will approximate the value of by finding where this line is . Step 7 : The length of the red vector can be calculated from the lines slope. The slope of the line, , is the rise over the run or . Solving for the red vector gives Step 8 : An alternate method is to find the point on the line, using the slope of the line through two points. . Solving for gives the Newton method. This is the same as adding the red vector to the initial . Step 9: Looking at the graph of the function it can be seen that the new values is close to the desired x value where . Step 10: Repeat the operation using this new value to get a better guess of the desired value.

Illustration of the Newton Method

In this applet you can enter a function in the entry box and provide an initial guess of by moving the point on the x-axis.

Newton Method for Finding Roots of a Function


For the following functions experiment with different values of the initial guess. = x^2 - 2 = 1/2 + sin(x) = x - cos(x) = 1/2 - nroot(x,3) If[ x< 0 , sqrt( -x) , sqrt(x)]