Step 1 :
After factoring out $ 10x $ we have:
$$ 10x^{3}-130x^{2}+400x = 10x ( x^{2}-13x+40 ) $$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:
$$ \color{blue}{ b = -13 } ~ \text{ and } ~ \color{red}{ c = 40 }$$Now we must discover two numbers that sum up to $ \color{blue}{ -13 } $ and multiply to $ \color{red}{ 40 } $.
Step 3: Find out pairs of numbers with a product of $\color{red}{ c = 40 }$.
PRODUCT = 40 | |
1 40 | -1 -40 |
2 20 | -2 -20 |
4 10 | -4 -10 |
5 8 | -5 -8 |
Step 4: Find out which pair sums up to $\color{blue}{ b = -13 }$
PRODUCT = 40 and SUM = -13 | |
1 40 | -1 -40 |
2 20 | -2 -20 |
4 10 | -4 -10 |
5 8 | -5 -8 |
Step 5: Put -5 and -8 into placeholders to get factored form.
$$ \begin{aligned} x^{2}-13x+40 & = (x + \color{orangered}{\square} )(x + \color{orangered}{\square}) \\ x^{2}-13x+40 & = (x -5)(x -8) \end{aligned} $$