Series resistors - Basic electronics circuits

Series resistors - what single resistor is the equivalent of two resistors connected in series?
There are numerous times in circuits where we want to replace two or more resistors with an equivalent single resistor, or, occasionally, replace a single resistor with two of them.

Figure 1 illustrates the case in which we are going to determine the single resistance, Req, that is the equivalent of the two resistors, R1 and R2, in series.

Req = R1 + R2