It is not an arithmetic sequence.
Work Step by Step
A sequence is arithmetic if $a_2−a_1=a_3−a_2=a_4−a_3=a_5−a_4=...=d$ For the given sequence we have that: $a_2-a_1=40-80=-40$ $a_3-a_2=20-40=-20$ It is not an arithmetic sequence.
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