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# OCR FP1 Roots of Polynomial Question watch

1. Hi really need some help with this FP1 question, unsure how to start it. Any hints or pointers would be much appreciated.

2. Replace all occurrences of x with u+2. So x^3 becomes (u+2)^3 and so on, then expand out.
3. (Original post by Hopple)
Replace all occurrences of x with u+2. So x^3 becomes (u+2)^3 and so on, then expand out.
Unless I've expanded incorrectly, I keep ending up with X^3 -5 = 0

No idea what to even do with that.

Sorry, still really confused!
4. (Original post by freshypot♥)
Unless I've expanded incorrectly, I keep ending up with u^3 -5 = 0

No idea what to even do with that.

Sorry, still really confused!
Changed x to u. You really can't find a solution for ?
5. (Original post by Mr M)
You really can't find a solution for ?
No, I meant that because the questions asks for the exact real root of the equation, and the solutions for u^3 -5 = 0 are all real but not exact, it's not giving me the correct answer, so I must be doing something fundamentally wrong.

I can find the solutions to u^3 = 5 though!
6. (Original post by freshypot♥)
No, I meant that because the questions asks for the exact real root of the equation, and the solutions for x^3 -5 = 0 are all real but not exact, it's not giving me the correct answer, so I must be doing something fundamentally wrong.

I can find the solutions to x^3 = 5 though!
I don't think you know what exact means.

is exact. Stop changing u (incorrectly) to x!
7. (Original post by freshypot♥)
Unless I've expanded incorrectly, I keep ending up with X^3 -5 = 0

No idea what to even do with that.

Sorry, still really confused!
Do you get u^3-5=0? If so, find u's real root, then substitute back that u=x-2
8. (Original post by Mr M)
I don't think you know what exact means.

is exact. Stop changing u (incorrectly) to x!
Ooops, sorry! I entered it into my calculator, and it gave me a decimal answer, therefore I thought it couldn't be exact. Bit stupid really.

So, my root answer would be ?
9. (Original post by freshypot♥)
Ooops, sorry! I entered it into my calculator, and it gave me a decimal answer, therefore I thought it couldn't be exact. Bit stupid really.

So, my root answer would be ?
Yes.
10. (Original post by Mr M)
Yes.
Thanks!
11. (Original post by Hopple)
Do you get u^3-5=0? If so, find u's real root, then substitute back that u=x-2
Thanks for your help!

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