supplementary
~ (to sth)
1 additional; extra 增补的; 补充的; 附加的
a supplementary payment, lecture, item 额外的付款、 讲座、 项目.
2 (mathematics 数) (of an angle) making a total of 180 with another angle (指角)补角的.
Who could have imagined, in the mid 1970s, for example, that by the end of the 20th century, computers would be as common in people's homes as TV sets?