Cos(a-π/6)*cos(a+π/6)=1/2(cos(a-π/6-a-π/6)+cos(a-π/6+a+π/6))=
=1/2(cos(-π/3)+cos2a)=1/2*cosπ/3+1/2cos2a=1/2*1/2+1/2*cos2a=
=1/4+1/2*cos2a
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Решение
a₆ = 5 a₅ = 21
a₆ = a₅ + d
d = a₆ - a₅ = 5 - 21 = - 16
a₅ = a₁ - 16*4
a₁ = a₅ + 64 = 21 + 64 = 85
a₁₀ = a₁ + 9d = 85 - 9*16 = - 59
Sn = (a₁ + a₁₀)*10 / 2 = (85 - 59)*5 = 130
Ответ ответ ответ отв5т ответ.