Use long division when a polynomial is divided by a binomial
Factor Theorem
factor P(x)x-a only if P(a)=0
Since the polynomial is in cubic form, we can graph it by finding its zeroes then sketching the curve along.
First, factor out the polynomial completely using Factor Theorem
Try out numbers that divide to the constant term which is 20 as the value of a then substitute into the polynomial
20 can be divided by: ±1,2,4,5,10,20
Trying out a=2:
P(x)
=
x3-2x2-19x+20
P(a)
=
a3-2a2-19a+20
Replace x with a
P(2)
=
23-2(2)2-19(2)+20
Substitute a=2
=
8-2(4)-38+20
=
8-8-38+20
=
-18
Since P(2)≠0, (x-2) is not a factor of the polynomial
Trying out a=1:
P(x)
=
x3-2x2-19x+20
P(a)
=
a3-2a2-19a+20
Replace x with a
P(1)
=
13-2(1)2-19(1)+20
Substitute a=1
=
1-2(1)-19+20
=
1-2-19+20
=
0
Since P(1)=0, (x-1) is the first factor of the polynomial
Find the second factor using Long Division.
Substitute components into the formula
P(Polynomial)
=
x3-2x2-19x+20
Divisor
=
x-1
=
Next, solve for each term of the quotient
First term of the quotient:
Divide the first term of the Polynomial by the first term of the Divisor. Place this above the Polynomial
x3÷x
=
x2
Multiply x2 to the divisor. Place this under the Polynomial
x2(x-1)
=
x3-x2
Subtract x3-x2 and write the difference one line below
Drop down -19x and repeat the process to get the second term of the quotient
Second term of the quotient:
Divide the first term of the bottom expression by the first term of the Divisor. Place this above the Polynomial
-x2÷x
=
-x
Multiply -x to the divisor. Place this one line below
-x(x-1)
=
-x2+x
Subtract -x2+x and write the difference one line below
Drop down 20 and repeat the process to get the third term of the quotient
Third term of the quotient:
Divide the first term of the bottom expression by the first term of the Divisor. Place this above the Polynomial
-20x÷x
=
-20
Multiply -20 to the divisor. Place this under the Polynomial
-20(x-1)
=
-20x+20
Subtract -20x+20 and write the difference one line below
Since r=0 and cannot be divided anymore, the quotient is x2-x-20
So far, we have factored the polynomial as (x-1)(x2-x-20)
Factor out the polynomial further by applying cross method
(x-1)(x2-x-20)
(x-1)(x+4)(x-5)
Notice that in this form, we can easily get the zeroes of the cubic curve.
Find the zeroes of the curve by equating the factorised polynomial to 0 and solving each value of x
(x-1)(x+4)(x-5)
=
0
First x value:
x-1
=
0
x-1+1
=
0+1
Add 1 to both sides
x
=
1
Second x value:
x+4
=
0
x+4-4
=
0-4
Subtract 4 from both sides
x
=
-4
Third x value:
x-5
=
0
x-5+5
=
0+5
Add 5 to both sides
x
=
5
Hence, the zeroes will be at x=1,-4,5
Finally, graph the cubic curve over the zeroes. Note that since the coefficient of the x3 term is positive, the curve should start from the bottom left