Framing a Formula

Step.1) Choose literals (Variables), such as a, x, A, X for the quantities you are working with, certain symbols are traditionally used to denote certain variables. However, the same symbol may be used to denote different variables in different contexts. for instance, ‘p’ is used to denote power in physics and principal in mathematics. ‘A’ is used to denote area in mensuration and amount in the context of interest.

Step.2) Use the rules or conditions relevant to the context to establish the relation between the variable.

Step.3) The subject of a formula is written on the left-hand side of the equality sign, while the other variables and constants are usually written on the right-hand side of the equality sign in a formula.


Example.-  If a man’s age is just triple of his son and if the age of father implies by F and son’s age is implies by S, then the relation between father & son’s age is  F = 3S

When one quantity is expressed in terms of odd other quantities, the quantity thus expressed is called the subject of the formula.

Area ‘A’ of the rectangle, of length ‘L’ and breadth ‘B’, is   A = L X B

In this formula, the area is expressed in terms of the other variables, namely the length and breadth, so area is the subject of the formula, but it may be necessary to express the length in terms of the area and the breadth. To do so, we can transform the relations by dividing both sides by ‘B’. Then, in the transformed formula –


        L  =  -----------


The subject is L, transforming a formula this way is called changing the subject of the formula. We have already made use of such transformations.

 If,  I = simple interest, P = principle , R = rate  ,  T =  Time

                 P  X  R  X  T

       I  =  -----------------


Given I and any two of the other variable, the third can be worked out by changing the subject of the formula, for example –

         I  X 100                I  X 100                 I  X  100

 P = ------------- , R  = ------------- ,  T  = -------------

          R X T                    P X T                     P X R