2.5 Apply the Remainder and Factor Theorems p. 120

How do you divide polynomials?What is the remainder theorem?

What is the difference between synthetic substitution and synthetic division?

What is the factor theorem?

When you divide a Polynomial f(x) by a divisor d(x), you get a quotient polynomial q(x) with a remainder r(x) written:

f(x) = q(x) + r(x)d(x) d(x)

The degree of the remainder must be less than the degree of the divisor!

Polynomial Long Division:

You write the division problem in the same format you would use for numbers. If a term is missing in standard form …fill it in with a 0 coefficient.

Example: 2x4 + 3x3 + 5x – 1 = x2 – 2x + 2

1503222 2342 xxxxxx

2x4 = 2x2

x2

2x2

1503222 2342 xxxxxx

2x2

+4x2-4x32x4-( )

- 4x27x3 +5x

7x3 = 7x x2

+7x

7x3 - 14x2 +14x-( )

10x2 - 9x -1

+10

10x2 - 20x +20-( )

11x - 21

remainder

The answer is written:

2x2 + 7x + 10 + 11x – 21 x2 – 2x + 2

Quotient + Remainder over divisor

Now you try one!

y4 + 2y2 – y + 5 =y2 – y + 1

Answer: y2 + y + 2 + 3 y2 – y + 1

2. (x3 – x2 + 4x – 10) (x + 2)

SOLUTION

Write polynomial division in the same format you use when dividing numbers. Include a “0” as the coefficient of x2 in the dividend. At each stage, divide the term with the highest power in what is left of the dividend by the first term of the divisor. This gives the next term of the quotient.

Multiply divisor by x3/x = x2.

x3 + 2x2

–3x2 + 4x Subtract. Bring down next term.Multiply divisor by –3x2/x = –3x.

– 3x2 – 6x

10x – 1 Subtract. Bring down next term.

Multiply divisor by 10x/x = 10.

10x + 20

– 30 remainder

x2 – 3x + 10 x + 2 x3 – x2 + 4x – 10)

quotient

x3 – x2 +4x – 10x + 2

= (x2 – 3x +10)+ – 30 x + 2

ANSWER

OR…

Use Synthetic Division (x3 – x2 + 4x – 10) (x + 2) Set x + 2 = 0. Solve for x x = −2 Use − 2 as the divisor for synthetic

division which is the same as synthetic substitution.

Synthetic division can be used to divide any polynomial by a divisor of the form “x −k”

Remainder Theorem:

If a polynomial f(x) is divisible by (x – k), then the remainder is r = f(k).

Now you will use synthetic division (like synthetic substitution)

f(x)= 3x3 – 2x2 + 2x – 5 Divide by x - 2

F(x) = x3 – x2 + 4x – 10 (x + 2)

– 2 1 −1 4 −10

– 2 6 – 20

1 – 3 10 – 30ANSWER

SOLUTION

f(x)= 3x3 – 2x2 + 2x – 5 Divide by x - 2 Long division results in ?...... 3x2 + 4x + 10 + 15

x – 2 Synthetic Division: f(2) = 3 -2 2 -5

23

6

4

8

10

20

15

Which gives you: 3x2 + 4x + 10 + 15 x-2

Synthetic Division

Divide x3 + 2x2 – 6x -9 by (a) x-2 (b) x+3 (a) x-2 1 2 -6 -9

2 1

2

4

8

2

4

-5

Which is x2 + 4x + 2 + -5 x-2

Synthetic Division Practice cont.

(b) x+3 12 -6 -9 -

31

-3

-1

3

-3

9

0

x2 – x - 3

Factor Theorem:

A polynomial f(x) has factor x-k if f(k)=0

note that k is a ZERO of the function because f(k)=0

Factoring a polynomial

Factor f(x) = 2x3 + 11x2 + 18x + 9 Given f(-3)=0

Since f(-3)=0 x-(-3) or x+3 is a factor So use synthetic division to find the

others!!

Factoring a polynomial cont.

211 18 9

-3 2

-6

5

-15

3

-9

0

(x + 3)(2x2 + 5x + 3)

So…. 2x3 + 11x2 + 18x + 9 factors to:

Now keep factoring-- gives you:

(x+3)(2x+3)(x+1)

Your Turn…Factor the polynomial completely given that x – 4 is a factor. f (x) = x3 – 6x2 + 5x + 12

SOLUTION

Because x – 4 is a factor of f (x), you know that f (4) = 0. Use synthetic division to find the other factors.

4 1 – 6 5 12

4 – 8 –12

1 – 2 – 3 0

Use the result to write f (x) as a product of two factors and then factor completely.

f (x) = x3 – 6x2 + 5x + 12 Write original polynomial.

= (x – 4)(x2 – 2x – 3) Write as a product of two factors.

= (x – 4)(x –3)(x + 1) Factor trinomial.

Your turn!

Factor f(x)= 3x3 + 13x2 + 2x -8 given f(-4)=0

(x + 1)(3x – 2)(x + 4)

Finding the zeros of a polynomial function

f(x) = x3 – 2x2 – 9x +18. One zero of f(x) is x=2 Find the others! Use synthetic div. to reduce the degree

of the polynomial function and factor completely.

(x-2)(x2-9) = (x-2)(x+3)(x-3) Therefore, the zeros are x=2,3,-3!!!

Your turn!

f(x) = x3 + 6x2 + 3x -10 X=-5 is one zero, find the others!

The zeros are x=2,-1,-5 Because the factors are (x-2)(x+1)(x+5)

How do you divide polynomials?

By long division What is the remainder theorem?

If a polynomial f(x) is divisible by (x – k), then the remainder is r = f(k).

What is the difference between synthetic substitution and synthetic division?

It is the same thing What is the factor theorem?

If there is no remainder, it is a factor.

AssignmentPage 124, 7, 9, 11-15 odd, 21-23 odd, 29-33 odd, 35- 37 all