a1+a5=2a3,a5=2a3-a1;
b1*b5=b3^2,b5=b3^2/b1;
且a1=b1>0,a3=b3>0,a1不等于a3
a5-b5=2a3-a1-b3^2/b1
=2a3-a1-a3^2/a1
=（2a3a1-a1^2-a3^2）/a1
=-（a1-a3）^2/a1