Finding n when sum is known

Sometimes the exam gives the total sum and asks for the number of terms. In such questions, students must set up the sum formula, substitute the known values, and solve the resulting equation for n. This often creates a quadratic equation, so neat algebra is important.

1. 1

   Enough for 1 mark

   Finding n when sum is known means using the sum formula to determine how many terms of an AP add up to a given total.

2. 2

   Add for 2 marks

   Example: If the sum of an AP is 30 and the terms are 2, 4, 6, 8, ..., then n = 5 because 2 + 4 + 6 + 8 + 10 = 30.

3. 3

   Add for 3 marks

   This often creates a quadratic equation, so neat algebra is important.