a--(1/a)=1
a^2--2+(1/a^2)=1
a^2+(1/a^2)=3
a^4+2+(1/a^4)=9
A^4--2+(1/a^4)=7
[a^2--(1/a^2)]^2=7
a^2--(1/a^2)=正负根号7.