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# P1 Question. watch

1. x^2, (8x+1) and (7x + 2), where x does not = 0, are the second, fourth and sixth terms respectively of an arithmetic series.

a) Find the value of x

I get a wrong answere, but I'm never sure on how to do these ones anyway so I'd be thankful for any help.

Cheers.

EDIT - Yes, I'm a ****ing idiot by writting the signs down wrong. Sorry for wasting your time for anyone who tried but I've now put up the correct question.
2. (Original post by Bebop)
x^2, (8x-1( and (7x + 2), where x does not = 0, are the second, fourth and sixth terms respectively of an arithmetic series.

a) Find the value of x

I get a wrong answere, but I'm never sure on how to do these ones anyway so I'd be thankful for any help.

Cheers.
nth term = a + (n-1)d

a + d = x^2
a + 3d = 8x - 1
a + 5d = 7x + 2

(a + 3d) - (a + d) = (8x - 1) - x^2
2d = 8x - 1 - x^2

(a + 5d) - (a + 3d) = (7x + 2) - (8x - 1)
2d = -x + 3

8x - 1 - x^2 = -x + 3
x^2 - 8x + 1 = x - 3
x^2 - 9x + 4 = 0

(x + (9/2))^2 - (81/4) + (16/4) = 0
(x + (9/2))^2 - (65/4) = 0
(x + (9/2))^2 = (65/4)
x + (9/2) = (±√65)/2
x = (±√65 - 9)/2

Don't have a calculator to check that... looks wrong to me because you usually don't get a question with irrational roots and a series question at the same time.
3. yeah , thats wrong.
4. mm i use different method

since 2,4,6th term common difference between these is 2d so it's same

8x-1-x^2=7x+2-(8x-1)

9x-4-x^2=0

get x = (-9±√65)/4

looks weird.. x can be 2 values.
5. (Original post by Bebop)
yeah , thats wrong.
What's the right answer? I got the same quadratic equation as mika1 - I'm surprised it doesn't factorise for a p1 question. Is it definitely an arithmetic series not geometric?
6. (Original post by Bebop)
yeah , thats wrong.
really? i got x^2 - 9x + 4=0
i noticed a mistake in mik1a's working.....when completing the square he put x+(9/2) instead of x-(9/2)....but the way you say it, seems like the answer is a long way off, not just a small mistake with the signs
7. N/m

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