# Newton-Raphson method

• Dec 30th 2009, 12:23 AM
sillylilly
Newton-Raphson method
An equation f(x) = 0 has two roots in the interval between 0 and 8.
Give the value or a description of two starting values for which the newton raphson method will fail to converge to either root of the equation.

cheers :)
• Dec 30th 2009, 12:27 AM
mr fantastic
Quote:

Originally Posted by sillylilly
An equation f(x) = 0 has two roots in the interval between 0 and 8.
Give the value or a description of two starting values for which the newton raphson method will fail to converge to either root of the equation.

cheers :)

Read practical considerations here: Newton's method - Wikipedia, the free encyclopedia
• Dec 30th 2009, 12:40 AM
sillylilly
Yes I was just reading that before, only I don't understand how I can relate that directly to my problem?
• Dec 30th 2009, 06:13 AM
HallsofIvy
If f(x)= 0 has two roots, then, by Rolle's theorem, there exist a value of x between those roots at which f' is 0. Quoting from the Wikipedia article mr. fantastic linked to
"4. It is clear from the formula for Newton's method that it will fail in cases where the derivative is zero. Similarly, when the derivative is close to zero, the tangent line is nearly horizontal and hence may "shoot" wildly past the desired root."