5*3.14*2=31.4dm(9+1)/2=5m9/2=4.5m(5*5-4.5*4.5)*3.14=14.915 我都初一了
5*3.14*2=31.4dm(9+1)/2=5m9/2=4.5m(5*5-4.5*4.5)*3.14=14.915