设a2 = a ,a1 = a - d ,a3 = a + d
b1 * b2 * b3 = (1/2)^(a + a - d + a + d) = (1/2)^(3a) = 1/8 -> a = 1
b1 + b2 + b3 = (1/2)^(1 - d) + 1/2 + (1/2)^(1 + d) = 1/2 * [(1/2)^(-d) + 1 + (1/2)^d] = 21/8
(1/2)^d = 4 , d = -2 或 (1/2)^d = 1/4 , d = 2
an = 2n - 3 或 an = 5 - 2n