# How many complete turns are needed? (arithmetic series)

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#1
Question

a = 12 cm and d = 0.05 cm
5000 = 1/2n(24 + 0.05(n - 1)) = 0.025n^2 + 11.975
n^2 = 199521
=> n = 446.68 which means there was 446 complete turns.

What did I do wrong?
0
1 month ago
#2
(Original post by TSR360)
Question

a = 12 cm and d = 0.05 cm
5000 = 1/2n(24 + 0.05(n - 1)) = 0.025n^2 + 11.975
n^2 = 199521
=> n = 446.68 which means there was 446 complete turns.

What did I do wrong?
5000/12 = 416, so it must be less than that?
You should have a linear term in n when you do the sum.
0
1 month ago
#3
(Original post by TSR360)
Question

a = 12 cm and d = 0.05 cm
5000 = 1/2n(24 + 0.05(n - 1)) = 0.025n^2 + 11.975
n^2 = 199521
=> n = 446.68 which means there was 446 complete turns.

What did I do wrong?
you missed out an n .... it should say 11.975n
1
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