Putting a2=x and √a2−1=y, we have
(a2+√a2−1)4+(a2−√a2−1)4
=(x+y)4+(x−y)4
=[4C0x4+4C1x3y+4C2x2y2+4C3xy3+4C4y4]+[4C0x4−4C1x3y+4C2x2y2−4C3xy3+4C4y4]
=2[4C0x4+4C2x2y2+4C4y4]
=2[x4+6x2y2+y4]
=2[(a2)4+(a2)2(√a2−1)2+(√a2−1)4]
=2[a8+6a4(a2−1)+(a2−1)2]
=2[a8+6a6−6a4+a4−2a2+1]
=2[a8+6a6−5a4−2a2+1]