Answer
$-10$
Work Step by Step
$S_{n}=\dfrac {2a_{1}+\left( n-1\right) d}{2}\times n\Rightarrow d=4-6=6-8=-2\Rightarrow S_{10}=\dfrac {2\times 8+\left( 10-1\right) \times \left( -2\right) }{2}\times 10=-10$
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