40 是视野角度（FOV），就像你睁大眼睛还是眯着眼睛看。
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，这一点在新收录的资料中也有详细论述
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decision, see the original announcement.，详情可参考新收录的资料
If you touch the vibrating string at its halfway point, that deadens the vibration along the string’s entire length, enabling you to hear it vibrating in halves.
printf("%-12s: 耗时%.3fms, 正确:%s\n",，详情可参考新收录的资料