We need a pair of numbers that multiply to \(-6\) and add to \(5\text{.}\) Note that we do care to keep track that they multiply to a negative product.
Factor Pair |
Sum of the Pair |
\(1\cdot(-6)\) |
\(-5\) (close; wrong sign) |
\(2\cdot(-3)\) |
\(14\) |
Factor Pair |
Sum of the Pair |
\(-1\cdot6\) |
\(5\) (what we wanted) |
\(-2\cdot3\) |
(no need to go this far) |
So \(z^2+5z-6=(z-1)(z+6)\text{.}\) To confirm that this is correct, we should check. Either by multiplying out the factored form:
\begin{align*}
(z-1)(z+6)\amp=(z-1)\cdot z+(z-1)\cdot6\\
\amp=z^2-z+6z-6\\
\amp\confirm{=}z^2+5z-6
\end{align*}
|
\(z\) |
\(-1\) |
\(z\) |
\(z^2\) |
\(-z\) |
\(6\) |
\(6z\) |
\(-6\) |
Or by evaluating the original expression at \(\substitute{1}\) and \(\substitute{-6}\text{:}\)
\begin{align*}
\substitute{1}^2+5(\substitute{1})-6\amp\wonder{=}0\amp(\substitute{-6})^2+5(\substitute{-6})-6\amp\wonder{=}0\\
1+5-6\amp\wonder{=}0\amp 36-30-6\amp\wonder{=}0\\
0\amp\confirm{=}0\amp 0\amp\confirm{=}0\text{.}
\end{align*}
Our factorization passes the tests.