Question

find the sum of first 21 terms of the series -11,-7,-3,1,5...

Answer 1

Answer:

here's the answer. Just apply the formula for sn and get the answer

Answer 2

The sum of first 21 term is 609

Step-by-step explanation:

Series: -11,-7,-3,1,5...

First term, a = -11

Common difference, d = 4

Sum formula, $S_n=\dfrac{n}{2}[2a+(n-1)d]$

Sum$=\dfrac{21}{2}(-11\times 2+(21-1)\times 4)$

Sum = 609

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