6) прямо по формуле
b1 = -8, q = 1/2,
S(6) = b1*(1 - q^6)/(1 - q) = -8(1 - 1/64)/(1/2) = -16*(1 - 1/64) = -16 + 16/64 = -15 3/4
7) (a^2-3a-1/a+3)*1/(a^2-1)*(a^2+a) = (a^3-3a^2+3a-1)/a*a(a+1)/((a-1)(a+1)) =
= [(a-1)^3*a(a+1)] / [a(a-1)(a+1)] = (a-1)^2 = (2,5 - 1)^2 = 1,5^2 = 2,25
8) Умножаем все на 10
-4x + 6 < 15(4x + 1)
-4x + 6 < 60x + 15
-15 + 6 < 64x
64x > -9
x > -9/64