Polynomial Graph Matching

Polynomial Graph Matching


IITM Quiz 1


2๏ธโƒฃ Polynomial Graph Matching

Question: Given $ p(x) = 0.3x^3(x^2 - 1)(x - 2)^2(x - 3) $, which figure represents the polynomial $ p(x) $?

Step-by-Step Explanation of the Solution ๐Ÿš€

Let’s break down how to analyze the polynomial and match it to the correct figure!

1. Finding the Roots (Where the Graph Touches or Crosses the x-axis) ๐ŸŒฑ

Given:

$$ p(x) = 0.3x^3(x^2 - 1)(x - 2)^2(x - 3) $$
  • $x^3$: Root at $x = 0$ with multiplicity 3 (triple root)
  • $(x^2 - 1)$: Roots at $x = 1$ and $x = -1$ (each single root)
  • $(x - 2)^2$: Root at $x = 2$ with multiplicity 2 (double root)
  • $(x - 3)$: Root at $x = 3$ (single root)

Summary Table:

RootMultiplicityBehavior at Root
$0$3Flattens, crosses axis
$1$1Crosses axis
$-1$1Crosses axis
$2$2Touches, bounces off
$3$1Crosses axis

2. Degree and End Behavior ๐ŸŽข

  • Expand the degree:
    • $x^3$ โ†’ degree 3
    • $(x^2 - 1)$ โ†’ degree 2
    • $(x - 2)^2$ โ†’ degree 2
    • $(x - 3)$ โ†’ degree 1
  • Total degree: $3 + 2 + 2 + 1 = 8$ (but let’s check carefully: $x^3$ ร— $x^2$ ร— $x^2$ ร— $x$ = $x^{3+2+2+1} = x^8$). However, the original solution says degree 9โ€”let’s clarify:
$$ (x^2 - 1) = x^2 - 1 \implies \text{degree 2} $$$$ (x - 2)^2 = x^2 - 4x + 4 \implies \text{degree 2} $$

So, multiplying:

  • $x^3$ (degree 3)
  • $(x^2 - 1)$ (degree 2)
  • $(x - 2)^2$ (degree 2)
  • $(x - 3)$ (degree 1)

Total degree: $3 + 2 + 2 + 1 = 8$.

End Behavior (since leading coefficient is positive, 0.3):

  • Even degree ($8$): Both ends go up ($x \to \pm\infty, p(x) \to +\infty$)
  • Odd degree ($9$): Left end down, right end up.

But since the polynomial is degree 8, both ends go up!

3. Behavior at Each Root ๐ŸŒˆ

  • Triple root ($x = 0$): The graph flattens and crosses the x-axis.
  • Double root ($x = 2$): The graph just touches the x-axis and turns around (bounces).
  • Single roots ($x = 1, -1, 3$): The graph crosses the x-axis sharply.

4. Putting It All Together: Matching the Figure ๐Ÿ–ผ๏ธ

  • Check for all roots at correct places.
  • Check that at $x = 0$, the graph flattens as it passes through.
  • At $x = 2$, the graph touches and bounces (does not cross).
  • At $x = 1, -1, 3$, the graph crosses the axis.
  • Both ends of the graph rise up (since degree is even and leading coefficient is positive).

If a figure shows all these features, that is the correct graph for $p(x)$!

๐ŸŽฏ Final Answer

Figure 1 represents the polynomial $ p(x) $! It has all the correct roots, the right behavior at each root, and the correct end behavior.


How Roots Are Found in the Solution ๐ŸŒฑ

Let’s see how the roots (also called zeros or x-intercepts) of the polynomial are determined step by step!

1. Start with the Polynomial

Given:

$$ p(x) = 0.3x^3(x^2 - 1)(x - 2)^2(x - 3) $$

2. Set the Polynomial Equal to Zero

To find the roots, set $ p(x) = 0 $:

$$ 0.3x^3(x^2 - 1)(x - 2)^2(x - 3) = 0 $$

A product is zero if any factor is zero!

3. Solve Each Factor Separately

  • $ x^3 = 0 $
    • Solution: $ x = 0 $
    • Multiplicity: 3 (because of the exponent 3)
  • $ x^2 - 1 = 0 $
    • Factor further: $ (x - 1)(x + 1) = 0 $
    • Solutions: $ x = 1 $, $ x = -1 $
    • Each has multiplicity 1
  • $ (x - 2)^2 = 0 $
    • Solution: $ x = 2 $
    • Multiplicity: 2 (because of the exponent 2)
  • $ x - 3 = 0 $
    • Solution: $ x = 3 $
    • Multiplicity: 1

4. Summary Table

FactorRootMultiplicityExplanation
$ x^3 $03$ x^3 = 0 $
$ x^2 - 1 $1, -11 each$ x^2 - 1 = 0 $
$ (x - 2)^2 $22$ (x - 2)^2 = 0 $
$ x - 3 $31$ x - 3 = 0 $

5. What Does Multiplicity Mean?

  • Multiplicity tells how many times a root repeats.
    • Odd multiplicity: The graph crosses the x-axis.
    • Even multiplicity: The graph touches and bounces off the x-axis.

In summary: To find the roots, set the polynomial equal to zero and solve each factor for $ x $. The exponents tell you the multiplicity (how many times each root occurs)!