Question

PLEASE PLEASE PLEASE HELP I NEED IT

What is the sum of the series?

Answer 1

Formula :

$n\left(a_1+\frac{d\left(n-1\right)}{2}\right)$

Plug in the values :

$20\left(1+\frac{-3\left(20-1\right)}{2}\right) = 550$

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Answer:550
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