0a]√(a²-r²)rdr=π∫[0--->a]√(a²-r²)rdr=π/2∫[0--->a]√(a²-r²)dr²=-(π/2)(2/3)(a²-r²)^(3/2) |[0--->a]=(π/3)a³则πa³/3=π/12解得：a=4开三次方...