Sum of an arithmetic sequence

Question

The direct formula of a sequence is given by Vn = 7 + 3n

 

Compute using the sum formula: (sum sign) 16 above and n=8 below. (7+3n)

 

The answer I got = 55 + 31/2 • 9 = 387

 

Is this correct?

 

Thank you!

Answer 1

7+3n, n=8,9,10,...16

there are 9 terms

31, 34, 37, ..., 55

S_n=(n/2)(a₁+a_n)

S_n=(9/2)(31+55)

S_n=(9/2)(86)

S_n=9*43

S_n=387

You are correct. You should put in the parentheses to avoid ambiguity.