设x²+y²=r,则4x²-5xy+4y²=5变换为4r-5xy=5,xy=(4r-5)/5,因为(x-y)²=x²+y²-2xy≥0,所以x²+y²≥2xy,则r=x²+y²≥2xy=2(4r-5)/5,整理得r≤10/3；因为(x+y)²...