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Find the sum of the series -11 - 3 + 5 + 13 + ... + 125 using the series formula.

Sagot :

devishri1977

Answer:

1026

Step-by-step explanation:

Common difference = d = 2nd term  - first term

 d = -3 - (-11) = -3 +11 = 8

First term = a = -11

First we need to find the number of terms in this series

[tex]a_{n} = 125\\\\a + (n- 1)d = a_{n}\\\\[/tex]

-11 + (n- 1) * 8 = 125

 (n-1)*8 = 125 + 11

 (n-1) * 8 = 136

n -1 = 136/8

n -1 = 17

  n = 17+1

 n = 18

[tex]S_{n} =\frac{n}{2}(a+a_{n})\\\\S_{18} =\frac{18}{2}(-11+125)\\\\[/tex]

   = 9 * 114

   = 1026

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