Factor the expression by grouping. First, the expression needs to be rewritten as $x^{2}+ax+bx+4$. To find $a$ and $b$, set up a system to be solved.
$$a+b=-5$$ $$ab=1\times 4=4$$
Since $ab$ is positive, $a$ and $b$ have the same sign. Since $a+b$ is negative, $a$ and $b$ are both negative. List all such integer pairs that give product $4$.
$$-1,-4$$ $$-2,-2$$
Calculate the sum for each pair.
$$-1-4=-5$$ $$-2-2=-4$$
The solution is the pair that gives sum $-5$.
$$a=-4$$ $$b=-1$$
Rewrite $x^{2}-5x+4$ as $\left(x^{2}-4x\right)+\left(-x+4\right)$.
$$\left(x^{2}-4x\right)+\left(-x+4\right)$$
Factor out $x$ in the first and $-1$ in the second group.
$$x\left(x-4\right)-\left(x-4\right)$$
Factor out common term $x-4$ by using distributive property.
$$\left(x-4\right)\left(x-1\right)$$
Steps Using the Quadratic Formula
Quadratic polynomial can be factored using the transformation $ax^{2}+bx+c=a\left(x-x_{1}\right)\left(x-x_{2}\right)$, where $x_{1}$ and $x_{2}$ are the solutions of the quadratic equation $ax^{2}+bx+c=0$.
$$x^{2}-5x+4=0$$
All equations of the form $ax^{2}+bx+c=0$ can be solved using the quadratic formula: $\frac{-b±\sqrt{b^{2}-4ac}}{2a}$. The quadratic formula gives two solutions, one when $±$ is addition and one when it is subtraction.
