（p的四次方+q的四次方）²-4p的四次方q的四次方
=p^8+2p^4q^4+q^8-4p^4q^4
=p^8-2p^4q^4+q^8
=（p^4-q^4）²
=【（p²-q²）（p²+q²）】²
=（p-a）²（p+q）²（p²+q²）²