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  4. Sum of first 20 terms of the series -10,-8,-6, ldots ldots is :

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Q. Sum of first 20 terms of the series $-10,-8,-6, \ldots \ldots$ is :

Sequences and Series

Solution:

first term $a=10$, common difference $d=2$
$S_{20}=\frac{20}{2}[-20+(20-1)(2)]=180$