原式=[a^6-a^(-6)]/[(a^4+1/a^4+1)(a-1/a)]
=(a²-1/a²)(a^4+1/a^4+1)/[(a^4+1/a^4+1)(a-1/a)]
=(a+1/a)(a-1/a)/(a-1/a)
=a+1/a
=(a²+1)/a