Help!!! Newton Raphson

[jimmyricuk]

i looking over my maths books and lecture notes for an exam i have next week but i am cofused about an element of newton raphson as there seems to be 2 paths to finding some answers

f(x) = 8x^4=0.45x^3-4.544x-0.1136, x=0.8

when looking at this i was given the equation "x - f(x)/f'(x) = xf'(x)-f(x)/f'(x)"

i have used this equation for this example but when i try to use it for an example such as :

f(x)=4x^3-2x^2-200x-50, x=7 i have to use the standard equation
(xn+1 = xn-(f(x)/f'(x)) or i get a completely wrong answer

the questions are worded exactly the same but if i try to use the same method for both i end up with a wrong answer for the latter, i'm really confused please help (ps answers are 0.8182 and 6.951)

[zaza]

Welcome to the Forums


Why don't you just use the correct equation, your "standard" equation?


zaza

[jimmyricuk]

as i say i have a test coming up, i do not know under what circumstances to use the different equations, i was wondering why the equation differs from the example, the questions are very similar appart from the numbers so i do not know why i would have to use a different method for each question

[zaza]

Use one equation for both questions, but use the right equation.


zaza

[jimmyricuk]

my point is i dont know when to use either equation, if i was to use one in an exam situation i wouldn't know if it was the right one to use or not

[VBAhack]

when looking at this i was given the equation "x - f(x)/f'(x) = xf'(x)-f(x)/f'(x)"

I think something is wrong with the stated equation. If x - f(x)/f'(x) = xf'(x)-f(x)/f'(x), then x = xf'(x) or 1 = f'(x'). This doesn't make any sense.

[zaza]

my point is i dont know when to use either equation, if i was to use one in an exam situation i wouldn't know if it was the right one to use or not

OK, to spell it out.



Use the second one. All the time.