设菱形的两条对角线分别为x、y,菱形的边长为a,因为菱形的两条对角线互相垂直平分,所以（x/2）^2+(y/2)^2=a^2,整理得,x^2+y^2=4a^2,由因为菱形的边长是它两条对角线的等比中项所以a^2=xy,把a^2=xy代入x^2+y^2=4a^2中...