S(10) = a + (n-1)d 34 = a + 9d ---- Eqn 1 Sum(20) = n/2(2a +(n-1)d) 710 = 20/2(2a + 19d) ----- Eqn 2 Solve these two equations and find the value of a and d and put the values in the first eqn (a + (n-1)d) to find the 25th term *n = the term *a = the starting number *d = the difference in the series for each number
