A8 = 3.8, a13 = 6.3 / Знайти S12

Question

A8 = 3.8, a13 = 6.3 / Знайти S12

Answer 1

A₈=a₁+7d=3.8
a₁₃=a₁+12d=6.3
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          5d=2.5, d=0,5
a₁+7·0,5=3,8,   a₁=3,8-3,5=0,3 ,a₁₂=6,3-0,5=5,8

S₁₂=(a₁+a₁₂)·d/2, S₁₂=(0,3+5,8)·12/2=6,3·6=37.8