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# C3 Numerical Methods help :S watch

1. I'm trying to do this question, part II.

I thought you would see a sign change when I do f(0.3) and f(0.4) but I don't? Can someone explain why you don't see a sign change here but sometimes you do?

I have the mark scheme here but I don't really get why you see a sign change when you do x - f(x)

Any help appreciated!

Mark scheme:
2. iam sorry man but i think there is something wrong with this question. where did you get this question from.
3. (Original post by victor_b1992)
iam sorry man but i think there is something wrong with this question. where did you get this question from.
This is from an official OCR past paper
4. well i do edexcel that might be y.
5. You would only get a change in sign if your equation was equal to zero. Here, it is equal to x.

Rearranging so that everything is on one side before substituting in 0.3 and 0.4 should give you the sign change you are expecting. Alternatively, you could compare 0.3 to f(0.3) and 0.4 to f(0.4). In one case, f(x) will be larger, and in the other, x will be larger. This indicates a root in the interval [0.3,0.4] (and it's the way the mark scheme has worded it).

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