a的平方-4a+1=0 等式两边同除a
a-4+1/a=0
a+1/a=4
因为a+1/a=4
所以(a+1/a)^2=4^2
a^2+2+1/a^2=16
a^2+1/a^2=16-2=14
(a-a分之一)的平方
=a^2-2+1/a^2
=a^2+1/a^2-2
=14-2
=12