令y=x^2+18x+30 原式＝〉y=2*根号（y+15）=> y^2=4(y+15) =>y^2-4y-60=0
=> y1=-6 y2=10
得到 x^2+18x+30=10 x^2+18x+30=-6
解得x1*x2=20 x3*x4=36
结果为x1*x2*x3*x4=20*36=720