Problem Study (Polynomial Division)

1.   If is a factor of , Find the values of and . 
Solution 
Let  .
Since is a factor of , the remainder when is divided by is 0.
By polynomial long division we can find the remainder.  

 
Hence we have . 
 
and .
Similarly, we can say , 
When , and
When , .

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 2.   If is divisible by , prove that .
Solution 
 Since is divisible by , The remainder when is divided by = is zero.
By polynomial long division,

and .
.

Miscellaneous Problems

Find the remainder when 3x³ - x² + 7x + 5 is divided by 3x + 2.

Given that 2x³ - x² - 2x + 3 = (Ax + B)(x - 1)(x + 2) + C(x - 1) + D, find the values of A, B, C and D. Hence or otherwise, deduce the remainder when

2x³ - x² - 2x + 3 is divided by x² + x - 2.

Solve the equation 2x³ + x² - 19x = 6, giving your answers to two decimal places where necessary.

The expression x³ + px² + qx + 6 has the same remainder when divided by

x + 1 and 2 - x. Given that the remainder when the expression is divided by x + 3 is -60, find the value of p and of q.

By remainder theorem,

remainder = f(-2/3)

= 3(-2/3)³ - (-2/3)² + 7(-2/3) + 5

= -1

Let x = 1

2 - 1 - 2 + 3 = D

D = 2

Let x = -2

-16 - 4 + 4 + 3 = -3C + 2

3C = 15

C = 5

Let x = 0

3 = B (-1)(2) -5 + 2

-2B = 6

B = -3

Sub any value other than 1, 2 & 0 into x

A = 2

2x³ - x² - 2x + 3 ≡ (Ax + B)(x - 1)(x + 2) + C(x - 1) + D

Since x² + x - 2 = (x - 1)(x + 2), (note the similarity?)

thus, remainder = C(x - 1) + D

= 5(x - 1) + 2

= 5x - 3

2x³ + x² - 19x = 6

let f(x) = 2x³ + x² - 19x - 6

(x - 3) f(3) = 0

(x - 3) is a factor of f(x)

let f(x) = (x - 3)(Ax² + Bx + C)

A = 2

C = 2

let x = 1

2 + 1 - 19 - 6 = -2(2 + B + 2)

-22 = -8 - 2B

B = 7

f(x) = (x - 3)(2x² + 7x + 2)

2x² + 7x + 2 = 0

x =

= -3.19 or -0.31 (3 s.f.)

x = -3.19 or -0.31 or 3

let f(x) = x³ + px² + qx + 6

by remainder theorem, f(-1) = f(2)

-1 + p - q + 6 = 8 + 4p + 2q + 6

p - q - 5 = 4p + 2q + 14

3p + 3q = 9 --------(1)

by remainder theorem, f(-3) = -60

-27 + 9p - 3q + 6 = -60

3q = 39 + 9p --------(2)

sub (2) into (1):

3p + 39 + 9p = -9

12p = -48

p = -4

q = 1

Factor Theorem အသံုးခ် ပုစာၦမ်ား

Example 2

Given that x² + x - 6 is a factor of 2x⁴ + x³ - ax + bx + a + b - 1, find the value of a and b.

x² + x - 6 = (x + 3)(x + 2)

Let f(x) = 2x⁴ + x³ - ax² + bx + a + b - 1

f(-3) = 2(-3)⁴ + (-3)³ - a(-3)² - 3b + a + b - 1 = 0

134 - 8a - 2b = 0

4a + b = 67 --------(1)

f(2) = 2(2)⁴ + 2³ - a(2)² + 2b + a + b - 1 = 0

39 - 3a + 3b = 0

a - b = 13 --------(2)

(1) + (2) : 5a = 80

a = 16

when a = 16, b = 3

Example 3

Use the factor theorem to find the value of k for which (a + 2b) where a does not = 0 and b does not = 0, is a factor of a⁴ + 32b⁴ + a³b(k + 3).

Solution



Let f(a) = a⁴ + 32b⁴ + a³b(k + 3)

f(-2b) = (-2b)⁴ + 32b⁴ + (-2b)³b(k + 3) = 0

48b⁴ - 8b⁴(k+ 3) = 0

8b⁴[6 - (k + 3)] = 0

8b⁴(3 - k) = 0

Since b does not = 0, 3 - k = 0

k = 3

Example 4

Determine the value of k for which x + 2 is a factor of (x + 1)⁷ + (2x + k)³.

Solution



Let f(x) = (x + 1) + (2x + k)

f(-2) = (-2 + 1) + (-4 + k) = 0

(k - 4) - 1 = 0

(k - 4) = 1

k - 4 = 1

k = 5

Example 5

Given that f(x) = x³ - x² - ax - b, where a and b are constants, has the factor x - 3 but has a remainder of 13x - 11 when divided by x + 4. Calculate the values of a and b.

Hence (i) factorise f(x) completely. (ii) Solve the equation 27x³ - 3ax = ax² + b.

Solution



f(3) = 0

3³ - 3² - 3a - b = 0

3a + b = 18 --------(1)

f(-4) = (-4)³ - (-4)² - a(-4) - b = 13(-4) - 11

4a - b = 17 --------(2)

sub (1) into (2): a = 5, b = 3

(i) Factorising.

f(x) = (x - 3)(x² + 2x + 1)

= (x - 3)(x + 1)²

(ii) 27x³ - 9x² - 3ax - b = 0

(3x)³ - (3x)² - a(3x) - b = 0

Let y = 3x

y³ - y² - ay - b = 0

(y - 3)(y + 1)² = 0 [from (1)]

y = 3, y = -1

x = 1, x = -1/3

Example 6

Show that the expression x³ + (k-2) x² + (k-7)x - 4 has a factor x+1 for all values of k .If the expression also has a factor x+2, find the value of k and the third factor.



Solution

Let f(x) = x³ + (k-2) x² + (k-7)x - 4

f(-1) =(-1)³ + (k-2)(-1)² + (k-7)(-1) - 4 = -1+ k - 2 - k + 7 - 4 = 0

Therefore (x+1) is a factor of f(x).

x + 2 is a factor of f(x). -------(given)

Therefore f(-2)=0

(-2)³ + (k-2)(-2)² + (k-7)(-2) - 4 = 0

-8 + 4k -8 - 2k +14 - 4 = 0

2k = 6

k = 3

Therefore f(x) = x³ + x² - 4x - 4

Therefore (x + 1) (x + 2) = x² + 3x + 2 is also a factor of f(x).

Therefore the third factor of f(x) is x - 2.



Example 7

Given that kx³ + 2x² + 2x + 3 is a factor of kx³ - 2x + 9, have a common factor, what are the possible values of k?



Solution

Let f(x) = >kx³ + 2x² + 2x + 3 and g(x) = kx³ - 2x + 9.

Let (x - c) be a common factor of both f(x) and g(x).Therefore ...

f(c) = 0

kc³ + 2c² + 2c + 3 = 0 ---------(1)



g(c) = 0

kc³ - 2c + 9= 0 ---------(2)



Subtracting equation(2) from (1),we have

2c² + 4c - 6 = 0

c² + 2c - 3 = 0

(c + 3) (c - 1) =0

c = -3 or c = 1



Substituting c = -3 or c = 1 in equation (2), we get

k = 5/9 when c = -3 and

k = -7 when c = 1