当n为偶数时,a^n+（1/a^n)=2
当n为奇数时,a^n+（1/a^n)=-2
[a+（1/a)]^2=4
a^2+2+(1/a^2)=4
a^2+(1/a^2)=2
[a+（1/a)]^3=-8
[a+（1/a)]^2 * [a+（1/a)]=-8
[a^2+2+(1/a^2)] * [a+（1/a)]=-8
a^3+a+2a+(2/a)+(1/a)+(1/a^3)=-8
a^3+3a+(3/a)+(1/a^3)=-8
a^3+(1/a^3)+3[a+(1/a)]=-8
a^3+(1/a^3)+3(-2)=-8
a^3+(1/a^3)-6=-8
a^3+(1/a^3)=-2