Question

Is this right? finding the sum of the arithmetic series. thanks.

Answer 1

\[s _{15} \] for 5+9+13+14

Answer 2

so the common difference is +4

an = a1 + (n-1)d
an = 5 + (n-1)(4)
a15 = 5 + (15-1)(4)
a15 = 5 + (14)(4)
a15 = 5 + 56
a15 = 61

Answer 3

then..

s16 = 16 (5 + 61)/2
s16 = 16(66)/2
s16 = 16(33)
s16 = 528

Answer 4

but that isnt right.. can you see where i am wrong? thanks.

Answer 5

@amistre64

Answer 6

you need sum of 15 terms, but you plugged in n=16 ??

plug in n=15 and try again :)

Answer 7

s15 = 15 (5 + 61)/2
....

Answer 8

Oh yes your right.. cant believe i missed that :) thanks.

Answer 9

I get 495 :)