sin(π/4+a)=1/4=sinπ/4cosa+cosπ/4sina即：√2/2(sina+cosa)=1/4等号两边同时平方得：1/2x(sina+cosa)�0�5=1/16sin�0�5a+2sinacosa+cos�0�5a=(1/16)x21+sin2a=1/8解得：sin2a=1/8-1=-7/8
