x,a1,a2,a3,y成等差数列,x,b1,b2,y成等比数列b1×b2 =xy(a1+a3)^2=(x+y)^2(a1+a3)^2/b1×b2=(x+y)^2/(xy)(x+y)^2-4xy=(x-y)^2≥0所以(x+y)^2≥4xy所以(x+y)^2/(xy)≥4所以(a1+a3)^2/b1×b2 的取值范围是[4,+∞）...