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More Lessons for Algebra

Common Core For Algebra

Student Outcomes

- Students understand the Fundamental Theorem of Algebra; that all polynomial expressions factor into linear terms in the realm of complex numbers. Consequences, in particular, for quadratic and cubic equations are understood.

Algebra 2 Engage NY Lesson 40, Part 1 of 2

Algebra 2 Engage NY Lesson 40, Part 2 of 2

Classwork

Opening Exercise

Write each of the following quadratic expressions as a product of linear factors. Verify that the factored form is
equivalent.

a. π₯^{2} + 12π₯ + 27

b. π₯^{2} β 16

c. π₯^{2} + 16

d. π₯^{2} + 4π₯ + 5

Example 1

Consider the polynomial π(π₯) = π₯^{3} + 3π₯^{2} + π₯ β 5 whose graph is shown to
the right.

a. Looking at the graph, how do we know that there is only one real
solution?

b. Is it possible for a cubic polynomial function to have no zeros?

c. From the graph, what appears to be one solution to the equation π₯^{3} + 3π₯^{2} + π₯ β 5 = 0?

d. How can we verify that this value is a solution?

e. According to the remainder theorem, what is one factor of the cubic expression π₯^{3} + 3π₯^{2} + π₯ β 5?

f. Factor out the expression you found in part (e) from π₯^{3} + 3π₯^{2} + π₯ β 5.

g. What are all of the solutions to π₯^{3} + 3π₯^{2} + π₯ β 5 = 0?

h. Write the expression π₯^{3} + 3π₯^{2} + π₯ β 5 in terms of linear factors.

Exercises 1β2

Write each polynomial in terms of linear factors. The graph of π¦ = π₯^{3} β 3π₯^{2} + 4π₯ β 12 is provided for Exercise 2.

- π(π₯) = π₯
^{3}+ 5π₯ - π(π₯) = π₯
^{3}β 3π₯^{2}+ 4π₯ β12

Example 2

Consider the polynomial function π(π₯) = π₯^{4} β 3π₯^{3} + 6π₯^{2} β 12π₯ + 8, whose
corresponding graph π¦ = π₯^{4} β 3π₯^{3} + 6π₯^{2} β 12π₯ + 8 is shown to the right. How
many zeros does π have?

a. Part 1 of the fundamental theorem of algebra says that this equation will
have at least one solution in the complex numbers. How does this align
with what we can see in the graph to the right?

b. Identify one zero from the graph.

c. Use polynomial division to factor out one linear term from the expression π₯^{4} β 3π₯^{3} + 6π₯^{2} β 12π₯ + 8.

d. Now we have a cubic polynomial to factor. We know by part 1 of the fundamental theorem of algebra that a
polynomial function will have at least one real zero. What is that zero in this case?

e. Use polynomial division to factor out another linear term of π₯^{4} β 3π₯^{3} + 6π₯^{2} β 12π₯ + 8.

f. Are we done? Can we factor this polynomial any further?

g. Now that the polynomial is in factored form, we can quickly see how many solutions there are to the original
equation π₯^{4} β 3π₯^{3} + 6π₯^{2} β 12π₯ + 8 = 0.

h. What if we had started with a polynomial function of degree 8?

Lesson Summary

Every polynomial function of degree π, for π β₯ 1, has π roots over the complex numbers, counted with multiplicity.

Therefore, such polynomials can always be factored into π linear factors, and the obstacles to factoring we saw
before have all disappeared in the larger context of allowing solutions to be complex numbers.

The Fundamental Theorem of Algebra:

- If π is a polynomial function of degree π β₯ 1, with real or complex coefficients, then there exists at least one number π (real or complex) such that π(π) = 0.
- If π is a polynomial function of degree π β₯ 1, given by π(π₯) = π
_{π}π₯^{π}+ π_{π-1}π₯^{πβ1}+ β¦ + π_{1}π₯ + π_{0}with real or complex coefficients π_{π}, then π has exactly π zeros π_{1}, π_{2}, β¦ , π_{π}(not all necessarily distinct), such that π(π₯) = π_{π}(π₯ β π_{1})(π₯ β π_{2}) β¦ (π₯ β π_{π}).

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