radnorgardens
New member
- Joined
- Dec 2, 2014
- Messages
- 26
Hello all, would really appreciate your assistance with the following:
Question:
The sum of an arithmetic series is 270. The common difference is 1 and the first term is 4. Calculate the number of terms in the series.
Formula: S20 = n/2(2a+(n-1)d
Where: n = number of terms, a is the first term, and d is the common difference.
So I know I need to isolate n by transposing the formule, but can't get the correct answer (I know it is 20). My attempt:
270 = n/2(2(4)+(n-1)1
270 = n/2(8+(n-1)1
270 x 2 = n(8+(n-1)1
540 = n(8+(n-1)1
540 x 1 = n(8+(n-1)
540 = n(8+(n-1)
540 = 8n+n2-n
540/8 = n+n2-n
67.5 = n2
8.23 = n
Thank you all.
Question:
The sum of an arithmetic series is 270. The common difference is 1 and the first term is 4. Calculate the number of terms in the series.
Formula: S20 = n/2(2a+(n-1)d
Where: n = number of terms, a is the first term, and d is the common difference.
So I know I need to isolate n by transposing the formule, but can't get the correct answer (I know it is 20). My attempt:
270 = n/2(2(4)+(n-1)1
270 = n/2(8+(n-1)1
270 x 2 = n(8+(n-1)1
540 = n(8+(n-1)1
540 x 1 = n(8+(n-1)
540 = n(8+(n-1)
540 = 8n+n2-n
540/8 = n+n2-n
67.5 = n2
8.23 = n
Thank you all.