a-1/a=3a=-0.5 代入到(a^4+1)/a^2得：[（-0.5)^4+1]/(-0.5)^2=(0.063+1)/0.25=4.252
(a-1/a)^2=9a2-2+(1/a)^2=9a^+(1/a)^2=11(a^4+1)/a^2=11
(a^4+1)/a^2 = a^2+1/a^2 = (a-1/a)^2 + 2 = 9+2 =11
