One of the formulas for the SUM of an A.P. is: , with: 

Substitute values for the variables and you should get, sum of A.P., or: .

NEVER go the route the other person suggests as it's very time consuming and EXTREMELY INEFFICIENT. What if there were 20, or 50, or 200 terms? 

Would she suggest the same method to you? Seeing what she usually does, I bet she would! 

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