General simplification
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$$x^{2} - 5 x + 4$$
$$\left(x - 4\right) \left(x - 1\right)$$
The perfect square
Let's highlight the perfect square of the square three-member
$$\left(x^{2} - 5 x\right) + 4$$
To do this, let's use the formula
$$a x^{2} + b x + c = a \left(m + x\right)^{2} + n$$
where
$$m = \frac{b}{2 a}$$
$$n = \frac{4 a c - b^{2}}{4 a}$$
In this case
$$a = 1$$
$$b = -5$$
$$c = 4$$
Then
$$m = - \frac{5}{2}$$
$$n = - \frac{9}{4}$$
So,
$$\left(x - \frac{5}{2}\right)^{2} - \frac{9}{4}$$
$$\left(x - 4\right) \left(x - 1\right)$$
Combining rational expressions
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$$x \left(x - 5\right) + 4$$
Assemble expression
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$$x^{2} - 5 x + 4$$
Rational denominator
[src]
$$x^{2} - 5 x + 4$$