Find the values for A and B in a geometric triangle. A and B refer to the side of the polygon that is opposite the angle from which you are calculating the proportional mean. If you are calculating the proportional mean of adjacent triangles, A and B are the same value, as the shared side is where you are calculating the proportion.
To find the proportional mean, enter A and B into the following equation: X=Sq(A*B). As you can see, if A and B are the same, then X will equal either A or B. So if you are calculating the proportional mean of A=12 and B=3, start by multiplying A and B to make 36. Then take the square of that, which is 6, resulting in X=6.
Check the proportional mean for accuracy, using the following formula. A/X=X/B. Using the numbers for the previous equation, the formula is 12/6=6/3. If the numbers equate, which in this case is 2=2, then the proportional mean is accurate.