a+1/a=2
a平方+1/a平方=(a+1/a)^2-2=4-2=2
a4次方+1/a的4次方=(a^2+1/a^2)^2-2=4-2=2
(a-1/a)=a^2+1/a^2-2=2-2=0
a-1/a=0