a^2-3a+1=0 b^2-3b+1=0 所以a,b是方程x^2-3x+1=0的两个跟 a+b=3,ab=1 a^2+b^2=(a+b)^2-2ab=7 a^-2+b^-2 =1/a^2+1/b^2 =(a^2+b^2)/(ab)^2 =7/1 =7 选B
