Step 1:  Recall the definition of the \tan

              DEFINITION: The tangent (tan) of an angle in a right triangle is a ratio. It is

                                the length of the opposite leg (opp) divided by the length of  

                                the adjacent leg (adj).

                                 \tan = \frac{opp}{adj}

Step 2: Take any one acute angle (A or B) and apply the \tan rule.

              EXAMPLE: Take A angle

                                  Opposite side of angle A = CB = a

                                  Adjacent side of angle A = AC = b

                                 \tan = \frac{a}{b}

Step 3: In the same way apply the   \tan rule to another acute angle.

            EXAMPLE: Take B angle

                                  Opposite side of angle B = AC = b

                                  Adjacent side of angle B = BC = a

                                 \tan = \frac{b}{a}