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A-level Mathematics/OCR/C2/Dividing and Factoring Polynomials

< A-level Mathematics‎ | OCR‎ | C2

Remainder TheoremEdit

The remainder theorem states that: If you have a polynomial f(x) divided by x + c, the remainder is equal to f(-c). Here is an example.

What will the remainder be if   is divided by x - 3?


The remainder is 74.


When you factor an equation you try to "unmultiply" the equation. The N-Roots Theorem states that if f(x) is a polynomial of degree greater than or equal to 1, then f(x) has exactly n roots, providing that a root of multiplcity k is counted k times. The last part means that if an equation has 2 roots that are both 6, then we count 6 as 2 roots.

The Factor TheoremEdit

The factor theorem allows us to check whether a number is a factor. It states:

A polynomial   has a factor x - c if and only if  .

For example:

Determine if x + 2 is a factor of  .

Since c is positive instead of negative we need to use this basic identity:


Now we can use the factor theorem.


Since the resultant is 0, (x+2) is a factor of  .

This means it is possible to re-state the polynomial in the form (x+2)( some linear expression of x).

So   = (x+2)(ax+b)

Expanding the right hand side we get :


Equating like terms we get :

2= a

2a+b = 3 and

2b = -2

Giving a= 2, b= -1 from the first and third equations and this works in the second, so

  = (x+2)(2x-1)