cos67.5cos22.5为什么等于1/2sin45
为什么等于1/2sin45
　cos67.5cos22.5 = (1/2)[cos(67.5+22.5)+cos(67.5-22.5)] ...... 积化和差公式 = (1/2)(cos90+cos45) = (1/2)(0+√2/2) = √2/4