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Roots of Quadratic Equations - The Discriminant

Hi, I am just on the Roots of Quadratics section and I have been trying to get my head round this example but I just can't figure it out.

I think there is a chance it could of been badly written but would someone please be able to walk me through from start to finish and tell me where the '>' sign came from.

Here is the example written as is. Spacings and everything.

Thanks very much in advance :^]

Example 1

For what range of values of k does the equation x^2+(2k+1)x+(k+1) = 0, have 2 real unequal roots?

Discriminant. = (2k + 1)2 - 4(k+1) > 0
4k^2 - 3 > 0

We draw the graph of y = 4k^2 - 3

*The graph drawn shows x @ '-√3/4' and '√3/4' and the vertex on y @ -3'

Can someone please walk me through this example?
How was the >0 and the '-√3/4' and '√3/4' worked out?
(edited 10 years ago)
Reply 1
Original post by Bysteven
Hi, I am just on the Roots of Quadratics section and I have been trying to get my head round this example but I just can't figure it out.

I think there is a chance it could of been badly written but would someone please be able to walk me through from start to finish and tell me where the '>' sign came from.

Here is the example written as is. Spacings and everything.

Thanks very much in advance :^]

Example 1

For what range of values of k does the equation x^2+(2k+1)x+(k+1) = 0, have 2 real unequal roots?

Discriminant. = (2k + 1)2 - 4(k+1) > 0
4k^2 - 3 > 0

We draw the graph of y = 4k^2 - 3

*The graph drawn shows x @ '-√3/4' and '√3/4' and the vertex on y @ -3'

Can someone please walk me through this example?
How was the >0 and the '-√3/4' and '√3/4' worked out?


The >0 relates to the fact that you need 2 distinct roots

4k23=04k^2 - 3 = 0 gives k2=34 k^2 = \dfrac{3}{4}
Reply 2
Thanks very much,
And the

Discriminant. = (2k + 1)2 - 4(k+1) > 0
4k^2 - 3 > 0

Is that just expansion of the brackets?
The discriminant of a quadratic polynomial tells us about how many roots there are and about their nature.

If f(x)=ax2+bx+cf(x) = ax^2 + bx + c, the discriminant is equal to b24acb^2 - 4ac

0 When solving the equation f(x)


You can derive all of these from the quadratic equation. If α1\alpha_1 and α2\alpha_2 are the roots:

α1,2=b±b24ac2a\alpha_{1,2} = \dfrac{-b \pm \sqrt{b^2 - 4ac}}{2a}

As an extension

(edited 10 years ago)
Reply 4
Original post by Bysteven
Thanks very much,
And the

Discriminant. = (2k + 1)^2 - 4(k+1) > 0
4k^2 - 3 > 0

Is that just expansion of the brackets?


yes
Reply 5
Thank you for your help! and how would I go about getting from

x^2 + (2k+1) + (k+1) = 0
to
= (2k+1)2 - 4(k+1) > 0

what model should I be following?
(edited 10 years ago)
Reply 6
Original post by TenOfThem
The discriminant is b24acb^2-4ac


and in this case due to the x2 a would be 1 hence the 4(k+1), I'm assuming it could be wrote as 4(1)(k+1)?
Reply 7
Original post by Bysteven
Thank you for your help! and how would I go about getting from

x^2 + (2k+1) + (k+1) = 0
to
= (2k+1)2 - 4(k+1) > 0

what model should I be following?


The discriminant is b24acb^2-4ac

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