1-3+3²-3³+...+3⁶

3⁰-3¹+3²-3³+...+3⁶

Геометрическая прогрессия

b1 = 1

q = -3

S7 = 1-3+3²-3³+...3⁶ = b1(q⁷-1)/q-1 = 3⁷-1/3-1 = 3⁷-1/2 = -2187-1/2 = -2188/2 = -1092