等比数列，则a1*a5=a2*a4=a3*a3=8/2=4 a3=2这三个数的积为a2*a3*a4=4*2=8
1/2 1 2 4 81/2 * 1 * 2 * 4 * 8 = 321 * 2 * 4 = 8
