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An=a1+(n-1)×d
n=3; n=4
a3=3,49
a4=12,58
d=?
{a1+(3-1)×d=3,49
{a1+(4-1)×d=12,58
{a1+2d=3,49
{a1+3d=12,58
a1+2d-a1-3d=3,49-12,58
-d=-9,09|×(-1)
d=9,09.
a1=a3-(n-1)×d
a1=3,49-2×9,09
a1=3,49-18,18
a1=-14,69
<span>6 минут 40 секунд, 4 мин 10 сек, 26мин 40 сек</span>
B5 = 86•(-2)^5 = -86•32 = -2752.