若cosa+sina=1/2,则：(cosa+sina)²=1/4即：cos²a+2sinacosa+sin²a=1/41+sin2a=1/4解得：sin2a=-3/4而cos(a-π/4)=cosa*cos(π/4) + sina*sin(π/4)=(根号2)/2 *(cosa+sina)=(根号2)/4所以：cos(a-π/...