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 x37x2+4x+12=0.

Solution

Use trial and error to find the first root:

  • x=1: (1)37(1)2+4(1)+12=17+4+12=100

x=2(2)37(2)2+4(2)+12=828+8+12=0

So x=2 is a root.

Divide the cubic by x2.

x2)x37x2+4x+12¯

x2x2)x37x2+4x+12¯x32x2

x2x2)x37x2+4x+12¯x32x205x2+4x+12¯

x25xx2)x37x2+4x+12¯x32x205x2+4x+12¯5x2+10x

x25xx2)x37x2+4x+12¯x32x205x2+4x+12¯5x2+10x06x+12¯

x25x6x2)x37x2+4x+12¯x32x205x2+4x+12¯5x2+10x06x+12¯6x+120¯

We have already shown that x=2 was a root of the cubic equation, so now we must solve x25x6=0 to get the remaining two roots.

By factorising, we have x25x6=(x6)(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)=x37x2+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+3x211x6=0.

Solution

Use trial and error to find the first root:

x=12(1)3+3(1)211(1)6=2+3116=120

x=22(2)3+3(2)211(2)6=16+12226=0

So x=2 is a root.

Use the polynomial factorisation method to pull out a factor of x2:

2x3+3x211x6=2x2(x2)+7x(x2)+3(x2)=(x2)(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±49244=7±254=7±54

So x=754=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+3x211x6.

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 x3x24x6=0.

Solution

Use trial and error to find the first root:

x=1(1)3(1)24(1)6=1146=100

x=2(2)3(2)24(2)6=8486=100

x=3(3)3(3)24(3)6=279126=0

So x=3 is a root.

Divide the cubic by (x3):

x3x24x6=x2(x3)+2x(x3)+2(x3)=(x3)(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 b24ac=22(4×1×2)=48=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)=x3x24x6.

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 x32x221x26=0 using the factorisation method of polynomial division.

Example 2

Prof. Robin Johnson finds all the roots of the equation 6x3+x213x3=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

Whiteboard maths

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