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# C3 Iteration Question watch

1. In the C3 Edexcel textbook, chapter 4 example 7

Use the iteration formula...x n+ 1 = √x^3n - 2xn + 5 / 3 to find an approximation for the root of the equation x^3 - 3x^2 + 5 = 0

Sorry it's not so clear, I'm not sure how to write the subscript for when n is underneath.

For those who have the book, please could you explain how they work out x3 and x4...then suddenly jump to x14, x16?

How am I meant to know what sort of interval to jump?

Thanks
2. put x0 in the equation, get the answer, then replace x0 in your original eqaution with 'ans' from your calculator, keep tapping equals to get x2 x3 x4... xn.
3. basically, you have to keep on plugging in the values you get until you see that the answer's pretty much the same, or as in the example, up to x16. They 'jumped' it because it would get too tedious to type out every single value. But, when you do it, you shouldn't.
4. (Original post by Wesssty)
put x0 in the equation, get the answer, then replace x0 in your original eqaution with 'ans' from your calculator, keep tapping equals to get x2 x3 x4... xn.
(Original post by unamed)
basically, you have to keep on plugging in the values you get until you see that the answer's pretty much the same, or as in the example, up to x16. They 'jumped' it because it would get too tedious to type out every single value. But, when you do it, you shouldn't.

Thanks guys. So...in the exam is it likely I will have to sit and tap until x16 ?
5. (Original post by gottastudy)
Thanks guys. So...in the exam is it likely I will have to sit and tap until x16 ?
they normally tell you where to go up till, which is normally x4. And normally, to prove that there's a root, you don't have to use the iteration formula (you can if you want to). Say they say, prove that there is a root at 0.1356 (or whatever). You can do f(1.3555) and f(1.3565), and state that the sign changes and that the curve is continuous.

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