How many terms of A.P. 18,15,12,... are needed to give the sum 45? Explain the double answer.

What is the explaination for the double answer?

Consider the terms of the A.P. 18, 15, 12, 9, ...

Here the first term, *a *= 18, *d *= -3;

We need to find the number of terms of the above A.P. so that the sum will give 45

The formula for sum of A.P.is

Thus, the sum of A.P. is

Thus, the sum of first 3 terms or the sum of first 10 terms will be 45.

But, Check your question for the term 'double answer'.

**
**