For our course, a = 1 in all cases. (iii) x2 + 12x + 27
14. The fundamental principle of counting states if one event has m outcomes and another event has n outcomes, then the total number of possible outcomes from doing both events = (m)(n) An expression for the total number of possible outcomes from doing two events is x2
+ 8x + 16
Find an expression, in terms of x , for the number of outcomes for each event.
By the end of this section you should be able to:
● factorise a quadratic trinomial expression with a negative constant
Quadratic (highest power is two)
a x2 + bx − c Trinomial (three terms)
Here, the constant (c) is negative.
Strategy 1: Cross method, as covered in the previous section
This method is similar to the one outlined in the previous section, however the factors of the constant term will be of opposite sign.
Worked example 1 Factorise fully the expression x2