Step 1 :
After factoring out $ 3x $ we have:
$$ 3x^{3}-54x^{2}+240x = 3x ( x^{2}-18x+80 ) $$Step 2 :
Step 2: Identify constants $ \color{blue}{ b }$ and $\color{red}{ c }$. ( $ \color{blue}{ b }$ is a number in front of the $ x $ term and $ \color{red}{ c } $ is a constant). In our case:
$$ \color{blue}{ b = -18 } ~ \text{ and } ~ \color{red}{ c = 80 }$$Now we must discover two numbers that sum up to $ \color{blue}{ -18 } $ and multiply to $ \color{red}{ 80 } $.
Step 3: Find out pairs of numbers with a product of $\color{red}{ c = 80 }$.
PRODUCT = 80 | |
1 80 | -1 -80 |
2 40 | -2 -40 |
4 20 | -4 -20 |
5 16 | -5 -16 |
8 10 | -8 -10 |
Step 4: Find out which pair sums up to $\color{blue}{ b = -18 }$
PRODUCT = 80 and SUM = -18 | |
1 80 | -1 -80 |
2 40 | -2 -40 |
4 20 | -4 -20 |
5 16 | -5 -16 |
8 10 | -8 -10 |
Step 5: Put -8 and -10 into placeholders to get factored form.
$$ \begin{aligned} x^{2}-18x+80 & = (x + \color{orangered}{\square} )(x + \color{orangered}{\square}) \\ x^{2}-18x+80 & = (x -8)(x -10) \end{aligned} $$