Find zeros of polynomial and factor it out, find the reals and complex numbers

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Discussion Overview

The discussion revolves around finding the zeros of the polynomial $$f(x) = 3x^2 + 2x + 10$$ using the quadratic formula, and understanding the presence of imaginary roots. Participants explore methods such as completing the square and the implications of the discriminant being negative.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant applies the quadratic formula and finds the roots to be $$x_{1,2} = \frac{-1 \pm i \sqrt{29}}{3}$$, noting the presence of the imaginary unit $$i$$ due to a negative discriminant.
  • Another participant explains that the discriminant $$\Delta = b^2 - 4ac$$ is negative, leading to the conclusion that the roots are complex, specifically $$\sqrt{-116}$$ which introduces the imaginary unit.
  • A third participant provides a method of completing the square, showing that the minimum value of the function is positive, indicating that there are no real roots and confirming the expectation of complex roots.
  • There is a discussion about the interpretation of the imaginary unit $$i$$ and its role in the context of the quadratic equation.

Areas of Agreement / Disagreement

Participants generally agree that the polynomial has no real roots and that the roots are complex. However, there are different methods and explanations presented for arriving at this conclusion, indicating a lack of consensus on the preferred approach.

Contextual Notes

Some participants express uncertainty about the steps involved in deriving the imaginary roots, particularly regarding the manipulation of the square root of negative numbers and the completion of the square method.

Who May Find This Useful

This discussion may be useful for students or individuals interested in understanding complex roots of polynomials, the application of the quadratic formula, and the method of completing the square in quadratic equations.

datafiend
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Hi all,
$$f(x) = 3x^2+2x+10$$

I recognized that this a quadratic and used the quadratic formula. I came up with $$-1/3+-\sqrt{29}/3$$.

But the answer has a $$i$$ for imaginary. When I was under the \sqrt{116}, I broke that down, but didn't realize there would be an $$i$$

Can someone explain that one to me?

Thanks
 
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datafiend said:
Hi all,
$$f(x) = 3x^2+2x+10$$

I recognized that this a quadratic and used the quadratic formula. I came up with $$-1/3+-\sqrt{29}/3$$.

But the answer has a $$i$$ for imaginary. When I was under the \sqrt{116}, I broke that down, but didn't realize there would be an $$i$$

Can someone explain that one to me?

Thanks

Hi!

$$\Delta=b^2-4ac=2^2-4 \cdot 3 \cdot 10=4-120=-116=116i^2$$

$$x_{1,2}=\frac{-b \pm \sqrt{\Delta}}{2a}=\frac{-2 \pm \sqrt{116i^2}}{6}=\frac{-2 \pm 2 i \sqrt{29}}{6}=\frac{-1 \pm i \sqrt{29}}{3}$$

So, $x_1=\frac{-1+ i \sqrt{29}}{3}$ and $x_2=\frac{-1- i \sqrt{29}}{3}$.
 
$i$ is just a name for $\sqrt{-1}$, the imaginary unit.

The quadratic is $3x^2 + 2x + 10 = 0$. Multiply both sides by $4\cdot 3 = 12$ to get

$$4 \cdot 3^2 x^2 + 4 \cdot 3 \cdot 2x + 120 = (2 \cdot 3 \cdot x)^2 + 2 \cdot (2 \cdot 3) \cdot (2) x + (2)^2 + \left ( -4 + 120 \right)$$

By completing the square, one gets

$$(2 \cdot 3 x + 2)^2 + 116 = 0$$

And solving for $x$ results

$$x = \frac{-2 \pm \color{red}{\sqrt{-116}}}{6}$$

But we know that $\sqrt{ab} = \sqrt{a}\sqrt{b}$, thus $\sqrt{-116} = \sqrt{-1}\cdot \sqrt{166} = \sqrt{-1} \cdot 2 \cdot \sqrt{29} = \boxed{i2\sqrt{29}}$ which is the desired numerator.
 
Last edited:
If we complete the square to write the function in vertex form as follows, we find:

$$f(x) = 3x^2+2x+10=3\left(x^2+\frac{2}{3}x+\frac{1}{9}\right)+10-3\cdot\frac{1}{9}=3\left(x-\left(-\frac{1}{3}\right)\right)^2+\frac{29}{3}$$

We can then see that:

$$f_{\min}=f\left(-\frac{1}{3}\right)=\frac{29}{3}>0$$

So, we see that for any real value of $x$, the given function is greater than zero, and thus has no real roots. Thus, we should expect the roots to be complex.
 

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