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# Edexcel Jan 2010 C1 q.10 watch

1. I dont quite understand what they do for part b.
f(x)= x^2+4kx+(3+11k)
given that the equation has no real roots, find the set of possible values for k.

Why can i not do the b^2-4ac thing... i did but gave me the wrong answer. I am resitting this so a but unsure on when it =0, >0 or <0.

Thanks xxxx
2. (Original post by Smile-Like-You-Mean-It)
I dont quite understand what they do for part b.
f(x)= x^2+4kx+(3+11k)
given that the equation has no real roots, find the set of possible values for k.

Why can i not do the b^2-4ac thing... i did but gave me the wrong answer. I am resitting this so a but unsure on when it =0, >0 or <0.

Thanks xxxx
The discriminant

2 real roots.

1 real repeated root.

No real roots.

Using the last one, you will get a quadratic in k, which you can solve.

EDIT: Accuracy at it's best, or should I say pedantry
3. (Original post by Smile-Like-You-Mean-It)
I dont quite understand what they do for part b.
f(x)= x^2+4kx+(3+11k)
given that the equation has no real roots, find the set of possible values for k.

Why can i not do the b^2-4ac thing... i did but gave me the wrong answer. I am resitting this so a but unsure on when it =0, >0 or <0.

Thanks xxxx
Were the answers K>3 or k<-1/4????
4. (Original post by boromir9111)
Were the answers K>3 or k<-1/4????
yeh, could you show me?
5. so would a=1
b=4k
c=3+11k
i pumped those into the discriminant and got the wrong answer.
6. (Original post by Smile-Like-You-Mean-It)
I dont quite understand what they do for part b.
f(x)= x^2+4kx+(3+11k)
given that the equation has no real roots, find the set of possible values for k.

Why can i not do the b^2-4ac thing... i did but gave me the wrong answer. I am resitting this so a but unsure on when it =0, >0 or <0.

Thanks xxxx
When you sub it in you get

expand brackets, in the end you can re-write it as

7. (Original post by Smile-Like-You-Mean-It)
yeh, could you show me?
The way i did it is, even though 0 is greater than b^2 - 4ac the graph CANNOT cross the x-axis, hence it has to be above the x-axis

so we got then

by now, everyone has told you how to get the discriminant in that form(if not, let me know )

so now, first, expand, we get

that was a bit messy to me, so i divided it by 4 everything to get

now, this can be solved.......4*-3 = -12......so that comes out to be this

(4k-12)(4k+1)......(k-3)(4k+1)-----> k =3 and k = -1/4

but we need it when k>0 and i would really draw this out(plot the points for k) much more useful and you will see that, it's only greater than zero when -1/4>k or when k>3......hope that helps! if you need further help, PM me
8. (Original post by Clarity Incognito)
The determinant

2 real roots.

1 real repeated root.

No real roots.

Using the last one, you will get a quadratic in k, which you can solve.
Not to be pedantic or anything, but you do realise that the determinant is for matrices...
The word you're looking for is the discriminant.
9. (Original post by Farhan.Hanif93)
Not to be pedantic or anything, but you do realise that the determinant is for matrices...
The word you're looking for is the discriminant.
You're right, that is very pedantic
10. (Original post by boromir9111)
You're right, that is very pedantic
Well, now that I think about it. I guess it was a reasonable comment as the determinant is something COMPLETELY different, as I'm sure clarity is aware.
11. pedantic: overly concerned with minute details.

Impossible!
12. Alright guys, take it easy......i was only saying, don't shoot the messenger
13. (Original post by rnd)
pedantic: overly concerned with minute details.

Impossible!
Exactly, thank you.
It's a little bit more than a minute detail, probably just a slip on clarity's part considering he's probably in FM mode right now...
14. Answer in Mark Scheme is:

-¼< k < 3
15. (Original post by Farhan.Hanif93)
Not to be pedantic or anything, but you do realise that the determinant is for matrices...
The word you're looking for is the discriminant.
AHAHAHAHAAAAAA

twas a typo, I was using matrices when I posted that (my excuse)
16. (Original post by Clarity Incognito)
AHAHAHAHAAAAAA

twas a typo, I was using matrices when I posted that (my excuse)
It's alright, you can thank me later
I thought it was because of something like that
17. i mix up determinant and discriminant too
18. (Original post by Pheylan)
i mix up determinant and discriminant too
HA, you two should start up a mixed-up maths society together.

Only joking. No offence intended. please don't hurt me
19. (Original post by boromir9111)
Alright guys, take it easy......i was only saying, don't shoot the messenger
You're not the messenger though as it was your thought?

Mathematical 'correctness' is important IMO. Without googling do you know what the discriminant means?
20. (Original post by clownfan)
You're not the messenger though as it was your thought?

Mathematical 'correctness' is important IMO. Without googling do you know what the discriminant means?
Means the subject on which a person is discriminated upon.

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