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aliya0001 [1]
3 years ago
15

The sum of the first n terms of an arithmetic sequence is n/2(4n + 20).

Mathematics
1 answer:
lesantik [10]3 years ago
8 0

Answer:

(a).

S_{n} =  \frac{n}{2} (4n + 20) \\  \\S _{n - 1} =  \frac{(n - 1)}{2} (4n - 4 + 20) \\  \\ S _{n - 1} =  \frac{(n - 1)}{2} (4n + 16) \\  \\S _{n - 1} = \frac{(n - 1)(4n + 16)}{2}  \\   \\  { \boxed{S _{n - 1} =  {2 {n}^{2} + 6n - 8}}} \\

(b).

from general equation:

S _{n - 1} =  \frac{(n - 1)}{2} (4n + 16)

first term is 4n

common difference:

16 =  \{(n - 1) - 1 \}d \\ 16 = (n - 2)d \\ d =  \frac{16}{n - 2}

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