The diameter of a sphere is four times the focal length of a concave mirror cut from it.

凹面镜如果是从圆球上切下来的，圆球的直径会是凹面镜焦距的四倍。

youdao

The surface became a dark mirror as we cut the motors, taking in the guttural whoops, whines, trills and clicks around us.

BBC: Taming the heart of the Peruvian Amazon

The Pensions Regulator has not yet approved this cut in Trinity Mirror's reduced contributions to the scheme.

BBC: Three pension tales: BT, Royal Mail and Trinity Mirror

In the mirror, she looked sharp enough to cut something, hard fixed lines beside her mouth, her eyes too big, her cheekbones jutting like knuckles under her skin, up to her elbows in dirty work, cleaning toilets.

NEWYORKER: Friendly Fire