1+6=2+5=3+4所以等比数列中a1a6=a2a5=a3a4所以原式=(a3a4)²=25
等比数列的性质a3a4=5=a1a6=a2a5所以a1a2a5a6=5x5=25
a1a6=a2a5=a3a4=5a1a2a5a6=25
25a3a4=a1a6=a2a5所以 a1a2a5a6=（a3a4)^2=25
