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a₈=-20
a₆=?
a₄=a₁+(4-1)r
a₈=a₁+(8-1)r
a₄=a₁+3r
a₈=a₁+7r
a₁=a₄-3r
a₁=a₈-7r
a₄-3r=a₈-7r
-8-3r=-20-7r
4r=-12
r=-3
a₁=-8-7r=-8+21=13
a₆=a₁+5r
a₆=13-15=-2
Odp:a₆=-2