Polynomials
Definition: p(x) = aₙxⁿ + aₙ₋₁xⁿ⁻¹ + ... + a₁x + a₀ (aₙ≠0). Degree = n.
Types: Linear (degree 1). Quadratic (degree 2). Cubic (degree 3).
Zero of a Polynomial: Value of x for which p(x) = 0.
Relationship Between Zeroes and Coefficients
Quadratic ax²+bx+c: Sum of zeroes = −b/a. Product = c/a.
Cubic ax³+bx²+cx+d: Sum = −b/a. Sum of products taken two at a time = c/a. Product = −d/a.
Division Algorithm: p(x) = g(x)·q(x) + r(x). Degree of r(x) < degree of g(x).
Finding Zeroes Graphically — Where the graph crosses the x-axis.
Geometric Meaning of Zeroes
Linear Polynomial p(x) = ax + b
The graph of a linear polynomial is a STRAIGHT LINE. It crosses the x-axis at EXACTLY ONE point. The x-coordinate of this point is the zero of the polynomial.
Example: p(x) = 2x − 6. The graph is a line through (0,−6) and (3,0). It crosses the x-axis at x = 3. So x = 3 is the zero. Check: p(3) = 2(3)−6 = 0 ✓.
'For a linear polynomial p(x) = ax+b, the zero is ALWAYS at x = −b/a.'
Quadratic Polynomial p(x) = ax² + bx + c
The graph is a PARABOLA. It can cross the x-axis at 0, 1, or 2 points.
| Zeroes | Graph | Discriminant |
|---|---|---|
| 2 distinct real zeroes | Parabola crosses x-axis at two points | Δ > 0 |
| 1 real zero (repeated) | Parabola TOUCHES x-axis at one point | Δ = 0 |
| No real zeroes | Parabola does NOT touch x-axis | Δ < 0 |
Example: p(x) = x² − 5x + 6. The parabola crosses x-axis at x = 2 and x = 3. These are the zeroes.
Example: p(x) = x² − 4x + 4 = (x−2)². The parabola touches the x-axis only at x = 2 (repeated zero).
Example: p(x) = x² + x + 1. The parabola stays entirely ABOVE the x-axis (a > 0, Δ < 0). No real zeroes.
Relationship Between Zeroes and Coefficients — Expanded
Quadratic Polynomial ax² + bx + c
If α and β are the zeroes:
- Sum: α + β = −b/a
- Product: αβ = c/a
Example 1: Find the zeroes of x² − 5x + 6 and verify the relationship. x² − 5x + 6 = (x−2)(x−3). Zeroes: α = 2, β = 3. Sum = 2+3 = 5. −b/a = −(−5)/1 = 5 ✓. Product = 2×3 = 6. c/a = 6/1 = 6 ✓.
Example 2: Find a quadratic polynomial with zeroes 4 and −3. Sum = 4+(−3) = 1. Product = 4×(−3) = −12. Polynomial = x² − (sum)x + product = x² − x − 12. Verify: x² − x − 12 = (x−4)(x+3). Zeroes are 4 and −3 ✓.
Example 3: If α and β are zeroes of 2x² − 5x + 3, find α² + β². α+β = 5/2, αβ = 3/2. α² + β² = (α+β)² − 2αβ = (25/4) − 2(3/2) = 25/4 − 3 = (25−12)/4 = 13/4.
Cubic Polynomial ax³ + bx² + cx + d
If α, β, γ are the zeroes:
- Sum: α+β+γ = −b/a
- Sum of products taken two at a time: αβ+βγ+γα = c/a
- Product: αβγ = −d/a
Example 4: Zeroes of x³ − 6x² + 11x − 6 are 1, 2, 3. Verify. Sum = 1+2+3 = 6. −b/a = −(−6)/1 = 6 ✓. αβ+βγ+γα = 1×2+2×3+3×1 = 2+6+3 = 11. c/a = 11/1 = 11 ✓. αβγ = 1×2×3 = 6. −d/a = −(−6)/1 = 6 ✓.
Division Algorithm
For polynomials p(x) and g(x) (g(x) ≠ 0), there exist unique q(x) and r(x) such that: p(x) = g(x) · q(x) + r(x), where degree of r(x) < degree of g(x).
'This is analogous to: Dividend = Divisor × Quotient + Remainder.'
Example 5: Divide x³ − 3x² + 5x − 3 by x² − 2. Find q(x) and r(x).
x − 3
──────────────
x²−2 ) x³ − 3x² + 5x − 3
x³ − 2x
─────────
−3x² + 7x − 3
−3x² + 6
────────────
7x − 9
q(x) = x − 3, r(x) = 7x − 9. Check: (x²−2)(x−3) + (7x−9) = x³−3x²−2x+6+7x−9 = x³−3x²+5x−3 ✓.
Example 6: On dividing x³ − 3x² + x + 2 by g(x), the quotient is x² − x + 1 and remainder is −2x + 4. Find g(x). p(x) = g(x)·q(x) + r(x) g(x) = [p(x) − r(x)]/q(x) = [(x³−3x²+x+2) − (−2x+4)] / (x²−x+1) = (x³−3x²+3x−2) / (x²−x+1) = x − 2. (Perform polynomial division.) So g(x) = x − 2.
AP Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Relationship between zeroes and coefficients | 3-4 | Very Common |
| Finding quadratic polynomial from zeroes | 2-3 | Very Common |
| Division algorithm | 3-4 | Common |
| Zeroes of cubic polynomial | 4 | Moderate |
| Graphical meaning of zeroes | 2-3 | Moderate |
Self-Test Questions
- Find the zeroes of x² − 2x − 8. (Answer: (x−4)(x+2) → zeroes: 4, −2)
- Find a quadratic polynomial whose zeroes are 3 and −5. (Answer: x² + 2x − 15)
- If α and β are zeroes of 3x² + 4x + 1, find 1/α + 1/β. (Answer: α+β=−4/3, αβ=1/3. 1/α+1/β = (α+β)/αβ = −4)
- Divide x³ + 2x² − 5x − 6 by x + 1. Find q(x) and r(x). (Answer: q(x)=x²+x−6, r(x)=0)
- Verify that 1, −2, 3 are zeroes of x³ − 2x² − 5x + 6. Check the relationship between coefficients and zeroes.
- Find the zeroes of 4x² − 4x − 3 and verify the relationship. (Answer: zeroes: 3/2, −1/2. Sum=1=−(−4)/4=1 ✓, Product=−3/4=c/a ✓)
