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22-03-2020
Mathematics

contestada

Find the number of terms, n, of the arithmetic series given a = 21, an = 39, and Sn = 210.
A.7
B. 8
C. 9
D.5

Respuesta :

Hrishii Hrishii

22-03-2020

Answer:

7

Step-by-step explanation:

[tex]S_n = \frac{n}{2} (a + a_n) \\ \\ 210 = \frac{n}{2} (21 + 39) \\ \\ 210 = \frac{n}{2} \times 60 \\ \\ 210 = n \times 30 \\ \\ n = \frac{210}{30} \\ \\ n = 7[/tex]

Hence, number of terms = 7

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