How to Solve Cubic Equation pdf

Introduction
Learn to Solve Cubic Equations
In mathematical terms, all cubic equations have
either one root or three real roots. The general
cubic equation is,
ax 3 + bx 2 + cx+d= 0
The coefficients of a, b, c and d are real or
complex numbers with a not equals to zero
(a ≠ 0). It must have the term x 3 in it, or else it
will not be a cubic equation.

Introduction
The coefficients of a, b, c and d are real or
complex numbers with a not equals to zero
(a ≠ 0). It must have the term x 3 in it, or else it
will not be a cubic equation. But any or all of b, c
and d can be zero.
The examples of cubic equations are,
No 1, x 3 + 3a 3 + 3 a 2 + a 3 – b=0
No 2, 4x 3 + 57=0
No 3, x 3 + 9x=0

Strategy to Solve Cubic
Equation
Unlike quadratic equation which may have no
real solution; a cubic equation always has at
least one real root. The prior strategy of solving
a cubic equation is to reduce it to a quadratic
equation, and then solve the quadratic by usual
means, either by factorizing or using a formula.
Always try to find the solution of cubic
equations with the help of the general equation,
ax 3 + bx 2 + cx+d= 0

Strategy to Solve Cubic
Equation
A cubic equation should, therefore, must be rearranged
into its standard form,
For example,
x 2 + 4x-1 = 6/x

Strategy to Solve Cubic
Equation
Step 1
You can see the equation is not written in
standard form, you need to multiply the ‘x’ to
eliminate the fraction and get cubic equation,
after doing so, you will end up with
x 3 + 4x 2 – x = 6

Strategy to Solve Cubic
Equation
Step 2
Then you subtract 6 from both sides in order to
get ‘0’ on the right side, so you will come up,
x 3 + 4x 2 – x- 6 = 0

Solving Cubic Equations with
the help of Factor Theorem
What is factor theorem? If you divide a
polynomial p(x) by a factor x – a of that
polynomial, then, you will end up with zero as
the remainder,
p(x) = (x – a)q(x) + r(x)

Solving Cubic Equations with
the help of Factor Theorem
If x – a is indeed a factor of p(x), then the
remainder after division by x – a will be zero.
Here is a problem,
With x= – 2 a solution.
p(x) = (x – a)q (x)
x 3 – 5x 2 – 2x+24 = 0

Solving Cubic Equations with
the help of Factor Theorem
If x – a is indeed a factor of p(x), then the
remainder after division by x – a will be zero.
Here is a problem,
With x= – 2 a solution.
p(x) = (x – a)q (x)
x 3 – 5x 2 – 2x+24 = 0

Solving Cubic Equations with
the help of Factor Theorem
Factor theorem says that if x = – 2 is a solution
of this equation, then x+2 is a factor of this
whole expression.

Solving Cubic Equations with
Step 1
the help of Factor Theorem
First, you need to look at the coefficients of the
original cubic equation, which are 1, -5, -
2 and 24.

Solving Cubic Equations with
the help of Factor Theorem
Step 2
Now multiply number (1) that just brought
down, by the known root -2, as a result is -2, you
mention the result in the other line, like

Solving Cubic Equations with
the help of Factor Theorem
Step 3
The numbers in the second column are added,
so giving us,

Solving Cubic Equations with
the help of Factor Theorem
Step 4
Then recently written number 7 is multiplied by
the known root, – 2,
As 14 comes as a result, you need to write it
down on the second row over the line,

Solving Cubic Equations with
the help of Factor Theorem
Step 5
Like previously the numbers in this column
added, (14 – 2 = 12)

Solving Cubic Equations with
the help of Factor Theorem
Step 6
And you need to go on with the process,

Solving Cubic Equations with
Step 7
the help of Factor Theorem
When you have zero at the bottom row, it gives
the confirmation that x = – 2 is a root of the
original cubic. At this stage, you got the first
three numbers in the bottom row as the
coefficients in the quadratic,
x 2 – 7x+12
Hence, you reduced your cubic to,

Solving Cubic Equations with
the help of Factor Theorem
(x+2)(x 2 – 7x + 12) =0
Step 8
After applying the quadratic term, the equation
comes like this,
(x +2) (x – 3) (x – 4) = 0
Resulting, you get the solution as x = -2 or 3 or 4.

Solving Cubic Equations with
the help of Factor Theorem
Another Example:
The equation is,
x 3 – 7x-6=0
Step 1 You can simply try x = – 1, after putting
the value of x, you will get,
(-1) 3 – 7(-1) -6

Solving Cubic Equations with
Step 2
the help of Factor Theorem
After applying the synthetic division, like above
example, you will take the coefficients of the
original cubic equation, which are 1, 0, -7 and -6,
you need to write down the know root x = -1 to
the right of the vertical line, giving us,

Solving Cubic Equations with
Step 3
the help of Factor Theorem
Multiply the brought down number 1 by the
known root x = -1, and put down the result (-
1) at the second row, like this,

Solving Cubic Equations with
Step 4
the help of Factor Theorem
The numbers of the second column are added to
the first column, giving us,

Solving Cubic Equations with
Step 5
the help of Factor Theorem
As you add more numbers to the second column
by following the synthetic division process, you
will come with,

Solving Cubic Equations with
Step 6
the help of Factor Theorem
Hence, the cubic reduced to quadratic,
(x+1)(x 2 -x- 6) =0
The factorized result is,
(x +1)(x – 3)(x + 2) = 0
You can get three solutions to the cubic
equation are x = -2, -1 or 3

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