2sina(sina+cosa)-1=2sina^2-1+2sinacosa=-cos(2a)+sin(2a) [sin2a=2sinacosa cos2a=1-2sina^2]=√2[sin2acosπ/4-cos2asinπ/4] 三角公式=√2sin(2a-π/4)sin(a-π/8)=3/5 则cos（a-π/8）=-4/5 由于5π/8<a{%<...