（1）∵1cm=0.01m，∴8cm=8×0.01m=0.08m；（2）∵1nm=10-9m，∴4.2nm=4.2×10-9m；（3）∵1m=106μm，∴1.7m=1.7×106μm；（4）∵1h=60min，∴0.5h=0.5×60min=30min；∵1h=3600s，∴0.5h=0.5×3600s=1800s．故答案为：（1）0.08；（2）4.2×10-9；（3）1.7×106；（4）30；1800．
