mono- 一
di- 二
tri- 三
tetra- 四
penta- 五

5 = penta
6 = hexa
7 = hepta
8 = octa
9 = nona
10 = deca
11 = hendeca
12 = dodeca
13 = trideca
14 = tetradeca
15 = pentadeca
16 = hexadeca
17 = heptadeca
18 = octadeca
19 = enneadeca
20 = icosa 一般后边加名词,如 double shifts.或当名词前缀,如 hexagon 六边形.

百度翻译  发音
http://fanyi.baidu.com/?aldtype=23#en/zh/double%20triple%20quadro%20penta%20hexa5%20%3D%20penta%0A6%20%3D%20hexa%0A7%20%3D%20hepta%0A8%20%3D%20octa%0A9%20%3D%20nona%0A10%20%3D%20deca%0A11%20%3D%20hendeca%0A12%20%3D%20dodeca%0A13%20%3D%20trideca%0A14%20%3D%20tetradeca%0A15%20%3D%20pentadeca%0A16%20%3D%20hexadeca%0A17%20%3D%20heptadeca%0A18%20%3D%20octadeca%0A19%20%3D%20enneadeca%0A20%20%3D%20icosa

为什么五在英语是penta?
七是hepta,六是hexa?不是seven和six吗?

five seven six的说法,发源於印欧语系,後为拉丁语等语言吸收,它是这个词源；而hepta等则是来自希腊语,可能用在各种特别称呼上.

在纪念长征胜利80周年大会上讲话的22次掌声来自哪里？（附现场同传视频+文本）
http://mp.weixin.qq.com/s?__biz=MzA3MTg1NjMwNg==&mid=2650821109&idx=2&sn=4d51077ea4d7e86256382b851a6665d7&chksm=84d3c828b3a4413e5dbc7b14f8999ab0ccd5e1f917db5add829fd3985330f95a35c85503cebf&mpshare=1&scene=1&srcid=1027luRG8XM3oAnv8qypuyEy#rd

英语1-20前缀