# How do you find the sum of the first 25 terms of an arithmetic sequence whose 7th term is −247 and whose 18th term is −49?

##### 1 Answer

The sum of the first 25 terms is

#### Explanation:

We know that

We also know that

**Equation (1):**

and

**Equation (2):**

Subtract Equation (2) from Equation (1).

**Equation (3): **

Substitute Equation (3) in Equation (1).

So

So the 25th term is given by

The sum

So