3. Getting the signs right !
If the last term of the trinomial is positive then the signs will be the same as
the middle term either both positive or both negative. If the last term of the
trinomial is negative then the signs will be different, one positive and one
4. Last terms: Trial and Error!
The product of the last terms always multiply together to give you the last term
of the trinomial. The sum of the product of the inside terms and the product of
the outside terms should add together to give you the middle term of the
trinomial. (Note: Be sure to include the signs of the last terms when
multiplying and adding)
Example 1: x² - 9x + 20
There is no common factor so we begin by setting up the first terms as follows:
(x )(x )
Next we get the signs right . Since the last term is positive, the signs will be
the same as the middle term which is negative .
(x - )(x - )
Finally we determine the last terms by trial and error. The last terms
multiplied together should equal 20 and when added together equal ¨C9. Therefore
our last terms will be ¨C4 and ¨C5.
(x - 4)(x - 5)
Example 2: x² - x - 20
Again there is no common factor so we begin by setting up our first terms as
(x )(x )
the signs right. Since the last term is negative, one term will be
positive and the other will be negative.
(x - )(x + )
Finally we determine the last term by trial and error. The terms multiplied
together must equal ¨C20 and when added together equal ¨C1. Therefore our last
terms will be ¨C5 and 4.
(x - 5)(x + 4)
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