6) sin (-7π) + 2 cos 31π/3 - tg 7π/4;

б) sin (-7π) + 2 cos (31π/3) - tg (7π/4) = sin (-7π) + 2 cos (10π + π/3) - tg (2π - π/4) = 0 + 2 cos (π/3) - (-tg (π/4)) = 0 + 2·(1/2) + 1 = 1 + 1 = 2

Ответ: 2