The index adds appreciably to the usefulness of the book.

索引明地增加了这本书的实用价。

Badly written questions limit the usefulness of questionnaires.

设计得不好，调查卷的用途就会打折扣。

The report is of potential usefulness to the government.

这个报告可能对政府有用。

I was persuaded of the usefulness of his new device.

我相了他的新装置的实用性。

The theory is of limited usefulness.

这个理论用途有限。

He has outlived his usefulness.

他已老而无用了。

Not only it can decide solely the lamp usefulness, but also available in mends light, sidelight, top light and the use of foot light.

它既可作主光灯用，也可作为补光、侧光、顶光、和脚光的使用。