解：x^2=(√7+√5)^2/4，y^2=(√7-√5)^2/4所以x^2+y^2=(7+5+7+5)/4=6.
x²+y²=1/4(√7+√5)²+1/4(√7-√5)²=1/4(7+2√35+5+7-2√35+5)=1/4×24=6
