sin 45° = $$\frac{\sqrt{2}}{2}$$
sin 420° = sin (360° + 60°) = sin 60° = $$\frac{\sqrt{3}}{2}$$
sin 45° sin 420° = $$\frac{\sqrt{2}}{2} \cdot \frac{\sqrt{3}}{2} = \frac{\sqrt{6}}{4}$$. Умноженное на √6: $$\frac{\sqrt{6}}{4} \cdot \sqrt{6} = \frac{6}{4} = \frac{3}{2}$$.