Given the inequality:
$$\left|{z - i}\right| + \left|{z + i}\right| < 4$$
To solve this inequality, we must first solve the corresponding equation:
$$\left|{z - i}\right| + \left|{z + i}\right| = 4$$
Solve:
$$x_{1} = -1.73205080756888$$
$$x_{2} = 1.73205080756888$$
$$x_{1} = -1.73205080756888$$
$$x_{2} = 1.73205080756888$$
This roots
$$x_{1} = -1.73205080756888$$
$$x_{2} = 1.73205080756888$$
is the points with change the sign of the inequality expression.
First define with the sign to the leftmost point:
$$x_{0} < x_{1}$$
For example, let's take the point
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$-1.73205080756888 + - \frac{1}{10}$$
=
$$-1.83205080756888$$
substitute to the expression
$$\left|{z - i}\right| + \left|{z + i}\right| < 4$$
$$\left|{z - i}\right| + \left|{z + i}\right| < 4$$
|I + z| + |z - I| < 4
one of the solutions of our inequality is:
$$x < -1.73205080756888$$
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x1 x2Other solutions will get with the changeover to the next point
etc.
The answer:
$$x < -1.73205080756888$$
$$x > 1.73205080756888$$