1)

2)

3)

Ответ:

1. 1/2;

2. √3/3;

3. 1.

Пошаговое объяснение:

1. cos(19π/3) = cos (6π + π/3) = cos(π/3) = 1/2;

2. tg(31π/6) = tg(5π + π/6) = tg(π/6) = √3/3.

3. ctg (13π/4) = ctg(3π + π/4) = ctgπ/4 = 1.