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Factor & Remainder Theorem (Cambridge O Level Additional Maths)
Revision Note
Factor Theorem
What is the factor theorem?
- The factor theorem is a useful result concerning the roots and factors of polynomials
- In the example below, the polynomial has three (linear) factors
- and
- and so it has the three roots and
- In the example below, the polynomial has three (linear) factors
- For a polynomial the factor theorem states that:
( is a root of )
and
Examiner Tip
- In an exam, the values of you'll need to find that make are going to be integers close to zero
- Try and first, then 2 and -2, then 3 and -3
- It is unlikely that you'll have to go beyond that
Worked example
(From part (ii) of our definition of factor theorem ...)
... if is a factor of then .
Since , is factor of .
Try first,
Since , is not a factor of .
Try ,
Since , is a factor of .
is another factor of .
is the third (linear) factor.
Once one factor is known, polynomial division could be used to find the others.
(In this case we were specifically asked to use factor theorem.)
Remainder Theorem
What is the remainder theorem?
- The factor theorem is actually a special case of the more general remainder theorem
- The remainder theorem states that when the polynomial is divided by the remainder is
- You may see this written formally as
- In polynomial division
- would be the result (at the top) of the division (the quotient)
- would be the remainder (at the bottom)
- is called the divisor
- In the case when and hence is a factor of – the factor theorem!
How do I solve problems involving the remainder theorem?
- If it is the remainder that is of particular interest, the remainder theorem saves the need to carry out polynomial division in full
- e.g. The remainder from is
- This is because if and
- If the remainder from a polynomial division is known, the remainder theorem can be used to find unknown coefficients in polynomials
- g. The remainder from is 8 so the value of p can be found by solving , leading to
- In harder problems there may be more than one unknown in which case simultaneous equations would need setting up and solving
- The more general version of remainder theorem is if is divided by then the remainder is
- The remainder is still found by evaluating the polynomial at the value of such that (the divisor is zero) but it is not necessarily an integer
Examiner Tip
- Exam questions will use formal mathematical language which can make factor and remainder theorem questions sound more complicated than they are
- Ensure you are familiar with the various terms from these revision notes
Worked example
The polynomial is given by , where and are integer constants.
When is divided by the remainder is 9.
When is divided by the remainder is 1.
Find the values of and .
Remainder theorem: " is the remainder when is divided by ".
when :
when :
Solving simultaneously,
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