5. 3 cos (arccos √2/2)

$$cos(arccos(\frac{\sqrt{2}}{2})) = \frac{\sqrt{2}}{2}$$

$$3 \cdot \frac{\sqrt{2}}{2} = \frac{3\sqrt{2}}{2}$$

Ответ: $$\frac{3\sqrt{2}}{2}$$