{an}成等比数列 an^=an-1 * an+1
lgan=lg(/an-1)*(/an+1)
=lg(/an-1) + lg(/an+1) /代表根号
=1/2lgan-1 + 1/2lgan+1
2lgan=lgan-1 + lgan+1
所以数列{lgan}是等差数列.