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sinαcosαtanα0°oo1o30°π/61/2√3/2√3/345°π/4√2/2√2/2160°π/3√3/21/2√390°π/210-120°2π/3√3/2-0.5-√3135°3π/4√2/2-√2/2-1150°5π/61/2-√3/2-√3/3180°π0-10270°3π/2-10-360°2π010
