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:
Root | Multiplicity | Behavior at Root |
---|---|---|
$0$ | 3 | Flattens, crosses axis |
$1$ | 1 | Crosses axis |
$-1$ | 1 | Crosses axis |
$2$ | 2 | Touches, bounces off |
$3$ | 1 | Crosses 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:
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
Factor | Root | Multiplicity | Explanation |
---|---|---|---|
$ x^3 $ | 0 | 3 | $ x^3 = 0 $ |
$ x^2 - 1 $ | 1, -1 | 1 each | $ x^2 - 1 = 0 $ |
$ (x - 2)^2 $ | 2 | 2 | $ (x - 2)^2 = 0 $ |
$ x - 3 $ | 3 | 1 | $ 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)!