mean quadratic error

例句：

1. 1.He meant to go, but he forgothis keys.
   他本打算去，但他忘了他的钥匙。
2. 2. The meaning of life isto live it fully.
   生活的意义是充分地生活。
3. 3. The mean streets were dark anddeserted.
   那些贫民窟的街道黑暗而荒凉。