Polynomial Graph Matching

Polynomial Graph Matching


IITM Quiz 1


2️⃣ 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 🚀

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) 🌱

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 🎢

  • Expand the degree:
    • $x^3$ → degree 3
    • $(x^2 - 1)$ → degree 2
    • $(x - 2)^2$ → degree 2
    • $(x - 3)$ → degree 1
  • Total degree: $3 + 2 + 2 + 1 = 8$ (but let’s check carefully: $x^3$ × $x^2$ × $x^2$ × $x$ = $x^{3+2+2+1} = x^8$). However, the original solution says degree 9—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 🌈

  • 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 🖼️

  • 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 $p(x)$!

🎯 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 🌱

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)!