设数列{an}的首项a1=a不等于1/4,且a的n+1项=1/2an（a为偶数）an+1/4（a为奇数）记bn=a2n-1-1/4.n=1.2.3.
1求a2.a3
a2n-1-1/4.n=1.2.3.
bn=a2n-1-1/4.n=1.2.3.
a2.a3 =1/2an