Solving Cubic Equations
Cubic Equations
A cubic equation is an equation of the form ax3+bx2+cx+d=0
By the Fundamental Theorem of Algebra, a cubic equation has either one or three real-valued solutions, or roots.
There is a formula to explicitly find the roots of any cubic equation, similar to the quadratic formula, but it's considerably more complicated. However, it is sometimes possible to find a solution by trial and error, or inspection. If one is found then we can immediately write down a linear factor of the cubic expression and use polynomial division to obtain an equation of the form (ax+b)(x2+cx+d)=0. Solving the quadratic equation x2+fx+g=0 will then give the last two roots (if they exist).
Worked Examples
Example 1
Find all the roots of x3−7x2+4x+12=0.
Solution
Use trial and error to find the first root:
- x=1: (1)3−7(1)2+4(1)+12=1−7+4+12=10≠0
x=2⇒(2)3−7(2)2+4(2)+12=8−28+8+12=0
So x=2 is a root.
Divide the cubic by x−2.
x−2¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯)x3−7x2+4x+12
x2x−2¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯)x3−7x2+4x+12x3−2x2
x2x−2¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯)x3−7x2+4x+12x3−2x2¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯0−5x2+4x+12
x2−5xx−2¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯)x3−7x2+4x+12x3−2x2¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯0−5x2+4x+12−5x2+10x
x2−5xx−2¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯)x3−7x2+4x+12x3−2x2¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯0−5x2+4x+12−5x2+10x¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯0−6x+12
x2−5x−6x−2¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯)x3−7x2+4x+12x3−2x2¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯0−5x2+4x+12−5x2+10x¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯0−6x+12−6x+12¯¯¯¯0
We have already shown that x=2 was a root of the cubic equation, so now we must solve x2−5x−6=0 to get the remaining two roots.
By factorising, we have x2−5x−6=(x−6)(x+1)
x=6 and x=−1 are roots of the quadratic, and hence of the cubic equation also.
Therefore, the roots of this cubic are
x1=2,x2=6,x3=−1
Here is the graph of f(x)=x3−7x2+4x+12.
As you can see, it crosses the axis three times, at the points x=2, x=6 and x=−1.
Example 2
Find all the roots of 2x3+3x2−11x−6=0.
Solution
Use trial and error to find the first root:
x=1⇒2(1)3+3(1)2−11(1)−6=2+3−11−6=−12≠0
x=2⇒2(2)3+3(2)2−11(2)−6=16+12−22−6=0
So x=2 is a root.
Use the polynomial factorisation method to pull out a factor of x−2:
2x3+3x2−11x−6=2x2(x−2)+7x(x−2)+3(x−2)=(x−2)(2x2+7x+3)
Now solve the quadratic 2x2+7x+3=0 to find the last two roots.
Using the quadratic formula:
x=−7±√72−(4×2×3)2×2=−7±√49−244=−7±√254=−7±54
So x=−7−54=−3 or x=−7+54=−12
Therefore, the roots of this cubic are
x=2,x=−3,x=−12
Here is the graph of f(x)=2x3+3x2−11x−6.
As you can see, it crosses the axis three times, at the points where x=2, x=3 and x=−12.
Example 3
Find all the roots of x3−x2−4x−6=0.
Solution
Use trial and error to find the first root:
x=1⇒(1)3−(1)2−4(1)−6=1−1−4−6=−10≠0
x=2⇒(2)3−(2)2−4(2)−6=8−4−8−6=−10≠0
x=3⇒(3)3−(3)2−4(3)−6=27−9−12−6=0
So x=3 is a root.
Divide the cubic by (x−3):
x3−x2−4x−6=x2(x−3)+2x(x−3)+2(x−3)=(x−3)(x2+2x+2)
The roots of the quadratic x2+2x+2 yield the remaining two roots of the cubic.
We will start by finding the discriminant of the quadratic to see if it has real roots.
We have the values a=1, b=2 and c=2, so the discriminant is b2−4ac=22−(4×1×2)=4−8=−4.
The discriminant is negative, so the quadratic has no real roots.
Therefore, this cubic only has one real root, x=3.
Here is the graph of f(x)=x3−x2−4x−6.
As you can see, it crosses the x-axis only once, at the point x=3.
Video Examples
Example 1
Prof. Robin Johnson finds all the roots of the equation x3−2x2−21x−26=0 using the factorisation method of polynomial division.
Example 2
Prof. Robin Johnson finds all the roots of the equation 6x3+x2−13x−3=0 using the long division method of polynomial division.
Workbook
This workbook produced by HELM is a good revision aid, containing key points for revision and many worked examples.
External Resources
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