Решите уравнение:

(2/3) ^x-1 + (2/3) ^x-2 = > 2.5

(2/3) ^ (x - 1) + (2/3) ^ (x - 2) ≥ 2.5

(2/3) ^x * (2/3) ^ (-1) + (2/3) ^x * (2/3) ^ (-2) ≥ 2.5

(2/3) ^x * (3/2 + 9/4) ≥ 2,5

(2/3) ^x * 3,75 ≥ 2,5

(2/3) ^x ≥ (2/3) ^1

Так как 0 < 2/3 < 1, то знак меняется на противоположный

x ≤ 1