11. 5 sin (arccos 0,8).

1) $$arccos(0.8) = a$$

2) $$sin(a) = \sqrt{1 - cos^2(a)}$$

3) $$sin(arccos(0.8)) = \sqrt{1 - 0.8^2} = \sqrt{1 - 0.64} = \sqrt{0.36} = 0.6$$

4) $$5 \cdot 0.6 = 3$$

Ответ: 3