1/xy/(1/x+1/y).(1/x^2-1/y^2)=1/[xy(1/x+1/y)]*(y^2-x^2)/x^2y^2=1/(x+y)*-(x-y)(x+y)/x^2y^2=-(x-y)/x^2y^2 a^2-4a+1=0两边除以a,a+1/a=4 (a^4+1)/a^2=a^2+1/a^2=(a+1/a)^2-2=16-2=14