Imaginary Roots and Vieta: 3a < 2+4c

In summary, the given equation will have complex roots if and only if 3a < 2 + 4c, regardless of the value of c. However, if c = 0, the equation will never have complex roots, making the condition 3a < 2 irrelevant. To determine when the equation has complex roots, one can set c = 1 and solve for a to find that the condition is 3a < 6.
  • #1
erisedk
374
7

Homework Statement


If both roots of the equation ax^2 + x + c - a = 0 are imaginary and c > -1, then:

Ans: 3a < 2+4c

Homework Equations


Discriminant < 0 for img roots
Vieta

The Attempt at a Solution


1-4(a)(c-a)<0
4ac > 4a^2 + 1
Minimum value of 4a^2 + 1 is 1 so
4ac>1

I can't think of anything beyond this and I have no idea how to incorporate c>-1 into my equations.
 
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  • #2
You mean "both roots are complex" not "imaginary", don't you? It is impossible for that equation to have imaginary roots.
 
  • #3
erisedk said:

The Attempt at a Solution


1-4(a)(c-a)<0
4ac > 4a^2 + 1

Okay, so far. What sort of inequality is that in relation to a?

... although, I don't see how you get that answer and c > 1 would make more sense. Are you sure you've got the question right?
 
Last edited:
  • #4
Both roots are complex is the same as both roots are imaginary, I think.

PeroK said:
Okay, so far. What sort of inequality is that in relation to a?

... although, I don't see how you get that answer and c > 1 would make more sense. Are you sure you've got the question right?

The question is correct, at least should be since it's a question from a test I took yesterday.
I don't have any idea what sort of inequality that is in relation to a.
 
  • #5
erisedk said:
Both roots are complex is the same as both roots are imaginary, I think.
The question is correct, at least should be since it's a question from a test I took yesterday.
I don't have any idea what sort of inequality that is in relation to a.

A complex number has real and imaginary parts. If both roots were imaginary, they would be ##\pm bi## with no real part.

It's a quadratic inequality in a. Never miss a quadratic!

Here's how I would prove the question is wrong:

Let c = 0 (which is > -1), so we have:

##ax^2 + x - a = 0##

The discriminant is ##4a^2+1## which is +ve, so this equation never has complex roots. And the condition 3a < 2 (=2 + 4c) is not relevant.

Ergo the question is wrong.
 
  • #6
Wait, why is the condition 3a<2 not relevant?
 
  • #7
erisedk said:
Wait, why is the condition 3a<2 not relevant?

The answer says that the equation has complex roots when 3a < 2 + 4c. In this case, when 3a < 2 (as c = 0).

But, with c = 0 the equation never has complex roots. Whether 3a < 2 or not has nothing to do with it.

As an exercise, try c = 1. You should be able to work out when the equation has complex roots and show that it not when 3a < 6
 
  • Like
Likes erisedk
  • #8
The equation always has complex roots for c=1.
That was neat. Thank you!
 

Related to Imaginary Roots and Vieta: 3a < 2+4c

1. What are imaginary roots?

Imaginary roots are solutions to a polynomial equation that involve the imaginary unit, which is denoted by i. The imaginary unit is defined as the square root of -1. Imaginary roots are necessary to solve certain equations that cannot be solved using only real numbers.

2. How do you find imaginary roots?

To find imaginary roots, you must first express the polynomial equation in the form ax^2 + bx + c = 0. Then, use the quadratic formula x = (-b ± √(b^2-4ac)) / 2a to solve for the roots. If the discriminant (b^2-4ac) is negative, the roots will be imaginary.

3. What is Vieta's formula?

Vieta's formula is a theorem that relates the coefficients of a polynomial to the sum and product of its roots. For a quadratic equation ax^2 + bx + c = 0, the sum of the roots is equal to -b/a and the product of the roots is equal to c/a.

4. How does Vieta's formula apply to the equation 3a < 2+4c?

In this equation, the coefficients are 3, 0, and -2. Using Vieta's formula, we can see that the sum of the roots is equal to 0/3 = 0 and the product of the roots is equal to -2/3. Therefore, the equation has no real roots, but it may have imaginary roots depending on the value of c.

5. What is the significance of imaginary roots and Vieta's formula in mathematics?

Imaginary roots and Vieta's formula are important in mathematics because they allow us to solve more complex equations and understand the relationships between the coefficients and roots of a polynomial. They are also used in various fields such as engineering, physics, and computer science to model and solve real-world problems.

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