a+1/a=4
平方 (a+1/a)^2=a^2+1/a^2+2=16
a^2+1/a^2=14
再平方
(a^2+1/a^2)^2=a^4+1/a^4+2=14*14=196
a^4+1/a^4=194
(a-1/a)^2=a^2+1/a^2-2=14-2=12
a+1/a=4
平方 (a+1/a)^2=a^2+1/a^2+2=16
a^2+1/a^2=14
再平方
(a^2+1/a^2)^2=a^4+1/a^4+2=14*14=196
a^4+1/a^4=194
(a-1/a)^2=a^2+1/a^2-2=14-2=12