The base model for realizing microstrip duals of capacitors and inductors is the 1/8 wavelength strip. A 1/8 wavelength transmission line will have a reactance of the line's characteristic impedance. So if you have a 1/8 (electrical wavelength) wavelength of RG-58/U it will measure 50 ohms on a bridge.
Measuring at one end with the other end open the reactance will be capacitive. With the end opposite of the measuring terminals shorted it will be inductive. Consider the open circuit model to be two parallel conductors like capacitor plates. Consider the short circuited model to be inductive with a distributed length of wire providing the inductance.
In the formula above you are calculating the length of a line (stub). For an 1/8 wavelength the cotangent of (Bl) is 1. If the line is 50 ohms then the reactance is 50 ohms and the 'j' operator provides an indicator the value is reactive. The minus sign indicates the reactance is capacitive.
The next formula is for the end of the line being shorted.
For a 1/8 wavelength line the tangent of (Bl) is 1, the 'j' operator is positive, thus the reactance value is positive. This indicates the line is inductive.
As you can probably see, the formulas allow for calculating the length of the line more than and less than 1/8 wavelength. The formula's final calculated value will tell you if your line is capacitive or inductive by the math sign. In general when using microstrip you try to use the shortest length as it takes up the least amount of real estate on your PCB.
Keep in mind this assumes lossless line and no propagation delay in the line. Losses will add equivalent series 'Resistance' and propagation delay will require physically shortening the strip length when compared to your calculated length.