Why are there only two roots of this cubic polynomial?

AI Thread Summary
The cubic polynomial x³ - 2.9x² + 2.8x - 0.9 has only two distinct roots due to the presence of a double root at x=1, which causes the graph to bounce off the x-axis rather than cross it. This behavior indicates a root of multiplicity two, while the root at x=0.9 is a simple root that crosses the x-axis. The graph near x=1 resembles a parabola, confirming the double root's nature. Understanding the graphical representation helps clarify the distinction between single and double roots. The polynomial's factored form is (x - 0.9)(x - 1)², emphasizing the root multiplicities.
PainterGuy
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Homework Statement
I'm not able to understand why I'm only getting two roots of cubic polynomial.
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Hi,

I was trying to find roots of the following cubic polynomial and there are only two roots. I believe there should be three roots. Could you please guide me why there are only two roots?

If you say that the "1" repeats itself as a root then I'd say the same could be said of "0.9". Thank you!
1618828164834.png


Source:
https://www.wolframalpha.com/input/?i=find+roots+x³-2.9x²%2B2.8x-0.9
 
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Why do you think the same could be said of the 0.9 root?
 
Try factorizing your cubic: x^3 - 2.9x^2 + 2.8x - 0.9 = (x - 1)(x - 0.9)(x - a). What is a?
 
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Thank you!

a=1
 
If you look at the graph, at x=1 the graph bounces off the x-axis. This is how you can find double roots visually, if the graph bounces off the axis and doesn't pass through it it's a double root (or a root of multiplicity 4,6,8 etc.The two ways you can prove this to yourself:
One is the sign of the function doesn't change as you pass through a double root but it does as you pass through a single root (you should try to prove this to yourself)

The other more graphical way is that a double root should look a lot like two single roots right next to each other. If you graphed (x-0.9)(x-1.00001)(x-0.99999) you would not visually be able to distinguish those two roots, the graph would go under the x-axis and pop back up over it in a span too small for the naked eye. A double root should look visually the same as that.
 
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Office_Shredder said:
If you look at the graph, at x=1 the graph bounces off the x-axis. This is how you can find double roots visually, if the graph bounces off the axis and doesn't pass through it it's a double root (or a root of multiplicity 4,6,8 etc.
I second what Office Shredder has said. Another way to think about this is that near x = 1, the cubic polynomial is acting like a quadratic polynomial; i.e. one whose graph is a parabola.

In factored form, the equation is ##y = (x - .9)(x - 1)^2##. If x is very close to 1, and hence relatively far from .9, the graph of the equation is ##y \approx .1(x - 1)^2##. This graph is a parabola, with vertex at (1, 0) and opening upward. For x values farther away from 1, the x - .9 factor exerts more of an influence.
 
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If you examined the graph under a microscope, the double root at x=1 looks and behaves like ##c_1(x-1)^2##, so it touches the y=0 axis and goes back up. Under the same microscope, at the root x=0.9, the graph looks like ##c_2(x-0.9)##, so it goes through the y=0 axis like a straight line. The two situations are not the same at all.
 
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