已知x>0,y>0,且x,a,b,y成等差数列,x,c,d,y成等比数列,r=(a+b)的平方/cd,则r的取值范围是?
r=(a+b)²/cd=(x+y)²/xy=(x²+2xy+y²)/xy=x/y+y/x+2≥2+2=4
故4≤r