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College Algebra Sixth Edition James Stewart  Lothar Redlin  Saleem Watson

College Algebra Sixth Edition James Stewart  Lothar Redlin  Saleem Watson. Polynomial and Rational Functions. 3. Complex Zeros and the Fundamental Theorem of Algebra. 3.6. Introduction. We have already seen that an n th-degree polynomial can have at most n real zeros.

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College Algebra Sixth Edition James Stewart  Lothar Redlin  Saleem Watson

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  1. College Algebra Sixth Edition James StewartLothar RedlinSaleem Watson

  2. Polynomial and Rational Functions 3

  3. Complex Zeros and the FundamentalTheorem of Algebra 3.6

  4. Introduction • We have already seen that an nth-degree polynomial can have at most n real zeros. • In the complex number system, an nth-degree polynomial has exactly n zeros. • Thus, it can be factored into exactly n linear factors. • This fact is a consequence of the Fundamental Theorem of Algebra, which was proved by the German mathematician C. F. Gauss in 1799.

  5. The Fundamental Theorem of Algebra and Complete Factorization

  6. Fundamental Theorem of Algebra • The following theorem is the basis for much of our work in: • Factoring polynomials. • Solving polynomial equations.

  7. The Fundamental Theorem of Algebra • Every polynomialP(x) = anxn+ an-1xn-1 + . . . + a1x + a0(n≥ 1, an ≠ 0) • with complex coefficients has at least one complex zero. • As any real number is also a complex number, the theorem applies to polynomials with real coefficients as well.

  8. Fundamental Theorem of Algebra • The Fundamental Theorem of Algebra and the Factor Theorem together show that a polynomial can be factored completely into linear factors—as we now prove.

  9. Complete Factorization Theorem • If P(x) is a polynomial of degree n ≥ 1, then there exist complex numbers a, c1, c2, . . . , cn(with a ≠ 0) such thatP(x) = a(x –c1) (x –c2 ) . . . (x –cn)

  10. Complete Factorization Theorem—Proof • By the Fundamental Theorem of Algebra, P has at least one zero—which we will call c1. • By the Factor Theorem, P(x) can be factored as: P(x) = (x –c1) · Q1(x) where Q1(x) is of degree n – 1.

  11. Complete Factorization Theorem—Proof • Applying the Fundamental Theorem to the quotient Q1(x) gives us the factorizationP(x) = (x –c1) · (x –c2) · Q2(x) where: • Q2(x) is of degree n – 2. • c2 is a zero of Q1(x).

  12. Complete Factorization Theorem—Proof • Continuing this process for n steps, we get a final quotient Qn(x) of degree 0—a nonzero constant that we will call a. • This means that P has been factored as:P(x) = a(x –c1)(x –c2) ··· (x –cn)

  13. Complex Zeros • To actually find the complex zeros of an nth-degree polynomial, we usually: • First, factor as much as possible. • Then, use the quadratic formula on parts that we can’t factor further.

  14. E.g. 1—Factoring a Polynomial Completely • Let P(x) = x3 – 3x2 + x – 3 • Find all the zeros of P. • Find the complete factorization of P.

  15. Example (a) E.g. 1—Factoring Completely • We first factor P as follows.

  16. Example (a) E.g. 1—Factoring Completely • We find the zeros of P by setting each factor equal to 0: P(x) = (x – 3)(x2 + 1) • Setting x – 3 = 0, we see that x = 3 is a zero. • Setting x2 + 1 = 0, we get x2 = –1; so, x = ±i. • Thus, the zeros of P are 3, i, and –i.

  17. Example (b) E.g. 1—Factoring Completely • Since the zeros are 3, i, and –i, the Complete Factorization of P is:

  18. E.g. 2—Factoring a Polynomial Completely • Let P(x) = x3 – 2x + 4. • Find all the zeros of P. • Find the complete factorization of P.

  19. Example (a) E.g. 2—Factoring Completely • The possible rational zeros are the factors of 4: ±1, ±2, and ±4. • Using synthetic division, we find that –2 is a zero, and the polynomial factors as:P(x) = (x + 2) (x2 – 2x + 2)

  20. Example (a) E.g. 2—Factoring Completely • To find the zeros, we set each factor equal to 0. • Of course, x + 2 = 0 means x = –2. • We use the quadratic formula to find when the other factor is 0.

  21. Example (a) E.g. 2—Factoring Completely • So, the zeros of P are –2, 1 + i, and 1 – i.

  22. Example (b) E.g. 2—Factoring Completely • Since the zeros are – 2, 1 + i, and 1 – i, the Complete Factorization of P is:

  23. Zeros and Their Multiplicities

  24. Zeros and Their Multiplicities • In the Complete Factorization Theorem, the numbers c1, c2, . . . , cnare the zeros of P. • These zeros need not all be different. • If the factor x –c appears k times in the complete factorization of P(x), we say that c is a zero of multiplicityk.

  25. Zeros and Their Multiplicities • For example, the polynomial P(x) = (x – 1)3(x + 2)2(x + 3)5has the following zeros: • 1 (multiplicity 3) • –2 (multiplicity 2) • –3 (multiplicity 5)

  26. Zeros and Their Multiplicities • The polynomial P has the same number of zeros as its degree. • It has degree 10 and has 10 zeros, provided we count multiplicities. • This is true for all polynomials, as we prove in the following theorem.

  27. Zeros Theorem • Every polynomial of degree n ≥ 1 has exactly n zeros—provided that a zero of multiplicity k is counted k times.

  28. Zeros Theorem—Proof • Let P be a polynomial of degree n. • By the Complete Factorization Theorem, P(x) = a(x –c1)(x –c2) ··· (x –cn)

  29. Zeros Theorem—Proof • Now, suppose that c is a zero of P other than c1, c2, . . . , cn. • Then, P(c) = a(c –c1)(c –c2) ··· (c –cn) = 0

  30. Zeros Theorem—Proof • Thus, by the Zero-Product Property, one of the factors c –cimust be 0. • So, c =ci for some i. • It follows that P has exactly the n zeros c1, c2, . . . , cn.

  31. E.g. 3—Factoring a Polynomial with Complex Zeros • Find the complete factorization and all five zeros of the polynomialP(x) = 3x5 + 24x3 + 48x

  32. E.g. 3—Factoring a Polynomial with Complex Zeros • Since 3x is a common factor, we have: • To factor x2 + 4, note that 2i and –2i are zeros of this polynomial.

  33. E.g. 3—Factoring a Polynomial with Complex Zeros • Thus, x2 + 4 = (x – 2i )(x + 2i ). • Therefore, • The zeros of P are 0, 2i, and –2i. • Since the factors x – 2i and x + 2i each occur twice in the complete factorization of P, the zeros 2i and –2i are of multiplicity 2 (or double zeros). • Thus, we have found all five zeros.

  34. Factoring a Polynomial with Complex Zeros • The table gives further examples of polynomials with their complete factorizations and zeros.

  35. E.g. 4—Finding Polynomials with Specified Zeros • Find a polynomial P(x) of degree 4, with zeros i, –i, 2, and –2 and with P(3) = 25. • Find a polynomial Q(x) of degree 4, with zeros –2 and 0, where –2 is a zero of multiplicity 3.

  36. Example (a) E.g. 4—Specified Zeros • The required polynomial has the form • We know that P(3) = a(34 – 3 · 32 – 4) = 50a = 25. • Thus, a = 1/2 . • So, P(x) = 1/2x4 – 3/2x2 – 2

  37. Example (b) E.g. 4—Specified Zeros • We require:

  38. Example (b) E.g. 4—Specified Zeros • We are given no information about Q other than its zeros and their multiplicity. • So, we can choose any number for a. • If we use a = 1, we get:Q(x) = x4 + 6x3 + 12x2 + 8x

  39. E.g. 5—Finding All the Zeros of a Polynomial • Find all four zeros ofP(x) = 3x4 – 2x3 – x2 – 12x – 4 • Using the Rational Zeros Theorem from Section 3.4, we obtain this list of possible rational zeros: ±1, ±2, ±4, ±1/3, ±2/3, ±4/3

  40. E.g. 5—Finding All the Zeros of a Polynomial • Checking them using synthetic division, we find that 2 and -1/3 are zeros, and we get the following factorization.

  41. E.g. 5—Finding All the Zeros of a Polynomial • The zeros of the quadratic factor are: • So, the zeros of P(x) are:

  42. Finding All the Zeros of a Polynomial • The figure shows the graph of the polynomial P in Example 5. • The x-intercepts correspond to the real zeros of P. • The imaginary zeros cannot be determined from the graph.

  43. Complex Zeros Come in Conjugate Pairs

  44. Complex Zeros Come in Conjugate Pairs • As you may have noticed from the examples so far, the complex zeros of polynomials with real coefficients come in pairs. • Whenever a + bi is a zero, its complex conjugate a –bi is also a zero.

  45. Conjugate Zeros Theorem • If the polynomial P has real coefficients, and if the complex number z is a zero of P, then its complex conjugate is also a zero of P.

  46. Conjugate Zeros Theorem—Proof • Let P(x) = anxn+ an-1xn-1 + . . . + a1x + a0where each coefficient is real. • Suppose that P(z) = 0. • We must prove that .

  47. Conjugate Zeros Theorem—Proof • We use the facts that: • The complex conjugate of a sum of two complex numbers is the sum of the conjugates. • The conjugate of a product is the product of the conjugates.

  48. Conjugate Zeros Theorem—Proof • This shows that is also a zero of P(x), which proves the theorem.

  49. E.g. 6—A Polynomial with a Specified Complex Zero • Find a polynomial P(x) of degree 3 that has integer coefficients and zeros 1/2 and 3 – i. • Since 3 – i is a zero, then so is 3 + i by the Conjugate Zeros Theorem.

  50. E.g. 6—A Polynomial with a Specified Complex Zero • That means P(x) has the form

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