Factorising Simple Quadratics
What is a quadratic expression?
- A quadratic expression is an algebraic expression where the highest power of the unknown is 2
- The general form for a quadratic expression is
- You will only come across quadratic expressions where
- Remember that or could have a value of 0
How do I factorise a quadratic expression?
- You can factorise a quadratic through inspection
- If c is positive then the factor pair must have either two positive factors or two negative factors
- A positive multiplied by a positive gives a positive
- A negative multiplied by a negative gives a positive
- E.g. Factorise
- Identify all the factor pairs of c
- In this example c = +4
- Factor pairs for this example include:
1, 4 or -1, -4
2, 2 or -2, -2
- Identify the factor pair that add together to give b
- In this example, b = +5
- Because of this we know that we are going to use a factor pair where both factors are positive
- The factor pair that adds to 5 is 1 and 4
- Write a pair of brackets each with x and one of the factors
- (x + 1)(x + 4)
- Identify all the factor pairs of c
How do I factorise a quadratic where c is positive but b is negative?
- If c is positive then the factor pair must have either two positive factors or two negative factors
- A positive multiplied by a positive gives a positive
- A negative multiplied by a negative gives a positive
- E.g. Factorise
- Identify all the factor pairs of c
- In this example, c = +15
- Factor pairs of +15 include:
1, 15 or -1, -15
3, 5 or -3, -5
- Identify the factor pair that add together to give b
- In this example, b = -8
- Because of this we know that we are going to use a factor pair where the factors are both negative
- The factor pair that adds to -8 is -3 and -5
- Write a pair of brackets each with x and one of the factors
- (x -3)(x - 5)
- Identify all the factor pairs of c
How do I factorise a quadratic where c is negative?
- If c is negative then the factor pair must have one positive factor and one negative factor
- A positive multiplied by a negative gives a negative
- E.g. Factorise
-
- Identify all the factor pairs of c
- In this example, c = -8
- Factor pairs of -8 include:
-1, +8 or +1, -8
+2, -4 or -2, +4
- Identify the factor pair that add together to give b
- In this example, b = -2
- The factor pair that adds to -2 is +2, -4
- Write a pair of brackets each with x and one of the factors
- (x + 2)(x - 4)
- Identify all the factor pairs of c
Examiner Tip
- As a check, expand your answer and make sure you get the same expression as the one you were trying to factorise.
Worked example
(a) Factorise .
Factorise the expression by inspection
We need two numbers that multiply to -21 and sum to -4
Identify all factor pairs of -21
+1, - 21
-1, +21
+3, -7
-3, +7
Identify the factor pair that sum to -4
+3, -7
Write down the brackets
(x + 3)(x - 7)