$$\left(x + 1\right) \left(x + 3\right)$$
General simplification
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$$y^{2} + 4 y + 3$$
The perfect square
Let's highlight the perfect square of the square three-member
$$\left(y^{2} + 4 y\right) + 3$$
To do this, let's use the formula
$$a y^{2} + b y + c = a \left(m + y\right)^{2} + n$$
where
$$m = \frac{b}{2 a}$$
$$n = \frac{4 a c - b^{2}}{4 a}$$
In this case
$$a = 1$$
$$b = 4$$
$$c = 3$$
Then
$$m = 2$$
$$n = -1$$
So,
$$\left(y + 2\right)^{2} - 1$$
Assemble expression
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$$y^{2} + 4 y + 3$$
Rational denominator
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$$y^{2} + 4 y + 3$$
Combining rational expressions
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$$y \left(y + 4\right) + 3$$
$$\left(y + 1\right) \left(y + 3\right)$$