question about ex-3.38
Merge remote-tracking branch 'huangz-sicp-answers/master'
Merge branch 'master' of https://github.com/huangz1990/SICP-answers
add explanation to ex-3.40, add 2 more possible results
Merge pull request #14 from ZHHY/master

修复练习 3.38 和 3.40 ，感谢 @ZHHY 的贡献。