Roots of Quadratic Equations (Edexcel International A Level Further Maths)

Revision Note

Author

Mark Curtis

Expertise

Maths

Sums & Products of Roots

How do I find the sum and product of the roots of a quadratic?

The quadratic equation $a x^{2} + b x + c = 0$ can be written as $x^{2} + \frac{b}{a} x + \frac{c}{a} = 0$
- by dividing by $a$ (assuming $a \neq 0$ )
If $α$ and $β$ are roots of the quadratic equation
- then the equation is also $(x - α) (x - β) = 0$ in factorised form
  - This works for real and complex roots
Both left-hand sides of the equations are identical
- $(x - α) (x - β) \equiv x^{2} + \frac{b}{a} x + \frac{c}{a}$
- Expand and collect the left-hand side
- $x^{2} - (α + β) x + α β \equiv x^{2} + \frac{b}{a} x + \frac{c}{a}$
- Equating coefficients gives
  - the sum of the roots is $α + β = - \frac{b}{a}$
  - the product of the roots is $α β = \frac{c}{a}$
Be careful: the sum of the roots has a negative sign
- The product does not

Exam Tip

These two relationships are not given in the Formulae Booklet, so must be learnt!

Worked Example

The quadratic equation $5 x^{2} - 30 x - 3 = 0$ has roots $α$ and $β$ .

Without solving the equation, find the value of:

(a) $α + β$

Either use $α + β = - \frac{b}{a} = - \frac{(- 30)}{5}$
Or first divide both sides of the equation by 5

$x^{2} - 6 x - \frac{3}{5} = 0$

Then use $x^{2} - (sum of roots) x + product of roots = 0$
The sum of the roots is the negative of the middle number

$α + β = - (- 6)$

$α + β = 6$

(b) $α β$

Either use $α β = \frac{c}{a} = \frac{(- 3)}{5}$
Or first divide both sides of the equation by 5

$x^{2} - 6 x - \frac{3}{5} = 0$

Then use $x^{2} - (sum of roots) x + product of roots = 0$
The product of the roots is the constant term on the end
No change of sign is needed

$α β = - \frac{3}{5}$

Expressions with Roots

How do I simplify expressions involving roots of a quadratic?

You can use algebra to write expressions in terms of $(α + β)$ and $α β$
Then use the fact that the equation $a x^{2} + b x + c = 0$ has roots $α$ and $β$ where
- $α + β = - \frac{b}{a}$
- $α β = \frac{c}{a}$
  - Substitute these into your expression to find its value

Which identities do I need to know?

You should know identities involving powers of products of roots
- $α^{2} β^{2} \equiv (α β)^{2}$
- $α^{3} β^{3} \equiv (α β)^{3}$
- and so on
You should know the identity for the sum of the squares of the roots
- $α^{2} + β^{2} \equiv (α + β)^{2} - 2 α β$
  - which comes from expanding $(α + β)^{2} \equiv α^{2} + β^{2} + 2 α β$
You should know the identity for the sum of the cubes of the roots
- $α^{3} + β^{3} \equiv (α + β)^{3} - 3 α β (α + β)$
  - which comes from the binomial expansion of ${(α + β)}^{3}$
  - $(α + β)^{3} \equiv α^{3} + 3 α^{2} β + 3 α β^{2} + β^{3} \equiv α^{3} + β^{3} + 3 α β (α + β)$
  - then rearranging

You should know how to use algebraic fractions (but do not need to learn these identities)
- $\frac{1}{α} \times \frac{1}{β} \equiv \frac{1}{α β}$
- $\frac{1}{α} + \frac{1}{β} \equiv \frac{α + β}{α β}$
- $\frac{α}{β} + \frac{β}{α} \equiv \frac{α^{2} + β^{2}}{α β}$
- and so on

Exam Tip

None of the identities are given in the Formulae Booklet, so you either need to learn them, or learn how to find them.

Worked Example

The quadratic equation $2 x^{2} + 10 x + 9 = 0$ has roots $α$ and $β$ .

Without solving the equation, find the exact value of

(a) $\frac{α}{β} + \frac{β}{α}$

Use $α + β = - \frac{b}{a}$ and $α β = \frac{c}{a}$ to find the sum and product of the roots

$α + β = - \frac{10}{2} = - 5$ and $α β = \frac{9}{2}$

Add the algebraic fractions $\frac{α}{β} + \frac{β}{α}$
Use a lowest common denominator of $α β$

$\begin{array}{rcl} \frac{α}{β} + \frac{β}{α} & = & \frac{α^{2}}{α β} + \frac{β^{2}}{α β} \\ = & \frac{α^{2} + β^{2}}{α β} \end{array}$

Now use the identity for the sum of the squares of the roots $α^{2} + β^{2} \equiv (α + β)^{2} - 2 α β$

$= \frac{{(α + β)}^{2} - 2 α β}{α β}$

Substitute in $α + β = - 5$ and $α β = \frac{9}{2}$

$\begin{array}{rcl} \frac{α}{β} + \frac{β}{α} & = & \frac{{(- 5)}^{2} - 2 (\frac{9}{2})}{(\frac{9}{2})} \\ = & \frac{25 - 9}{\frac{9}{2}} \\ = & 16 \div \frac{9}{2} \end{array}$

$\begin{array}{rcl} \frac{α}{β} + \frac{β}{α} & = & \frac{32}{9} \end{array}$

(b) $α^{3} + β^{3}$

Either use $α^{3} + β^{3} \equiv (α + β)^{3} - 3 α β (α + β)$ if learnt
Or start by expanding the binomial ${(α + β)}^{3}$

$(α + β)^{3} = α^{3} + 3 α^{2} β + 3 α β^{2} + β^{3}$

Factorise fully the middle two terms

$(α + β)^{3} = α^{3} + 3 α β (α + β) + β^{3}$

Make $α^{3} + β^{3}$ the subject

$α^{3} + β^{3} = (α + β)^{3} - 3 α β (α + β)$

Substitute in $α + β = - 5$ and $α β = \frac{9}{2}$

$\begin{array}{rcl} α^{3} + β^{3} & = & (- 5)^{3} - 3 (\frac{9}{2}) (- 5) \\ = & - 125 + \frac{135}{2} \end{array}$

$α^{3} + β^{3} = - \frac{115}{2}$

-57.5 is also accepted

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Roots of Quadratic Equations (Edexcel International A Level Further Maths)

Revision Note

Sums & Products of Roots

How do I find the sum and product of the roots of a quadratic?

Exam Tip

Worked Example

Expressions with Roots

How do I simplify expressions involving roots of a quadratic?

Which identities do I need to know?

Exam Tip

Worked Example

You've read 0 of your 0 free revision notes

Get unlimited access

Join the 100,000+ Students that ❤️ Save My Exams

Author: Mark Curtis