专题:cos
用半角公式也可倍角公式推得：cos45=2(cos22.5)²-1=1-2(sin22.5)²，解得sin22.5=[√（2-√2）]/2，cos22.5=[√（2+√2）]/2。
三角函数值表
| 角α | 弧度制 | sinα | cosα | tanα |
| 0° | o | o | 1 | o |
| 30° | π/6 | 1/2 | √3/2 | √3/3 |
| 45° | π/4 | √2/2 | √2/2 | 1 |
| 60° | π/3 | √3/2 | 1/2 | √3 |
| 90° | π/2 | 1 | 0 | - |
| 120° | 2π/3 | √3/2 | -0.5 | -√3 |
| 135° | 3π/4 | √2/2 | -√2/2 | -1 |
| 150° | 5π/6 | 1/2 | -√3/2 | -√3/3 |
| 180° | π | 0 | -1 | 0 |
| 270° | 3π/2 | -1 | 0 | - |
| 360° | 2π | 0 | 1 | 0 |
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