(1.5-r)^2*3.14*5=r^2*3.14*5
(1.5-r)^2=r^2
2.25-3r+r^2=r^2
3r=2.25
r=0.75
还是
1.5-r^2*3.14*5=r^2*3.14*5
2r^2*3.14*5=1.5
3.14r^2=0.15
r^2=15/314
r=正负（√4710）/314