Step 1: Note down the given terms, and recall the formula for sum of n terms of an arithmetic sequence

             NOTE: The sum of  n terms of an arithmetic sequence

                           [math] S_n = \frac{n}{2} [2a + (n- 1)d][/math]  

             Where first term ‘a’ and common difference ‘d’ and n =  number of terms

Step 2:  Setup an equation by substituting all known values in the formula in two cases

             EXAMPLE: Case 1:  Fist term a = a

                                            Common difference d = 3

                                                    n = 50 terms



Step 3: Solve the equation (linear equation) to get the unknown values

            NOTE: Solve the linear equations