sin315-cos135+2sin570的值是若sin20cos50=a,则sin50sin70的值是什么

sin315-cos135+2sin570
=sin(-45°)-cos135°+2sin(-150°)
=-sin45°-cos135°-2sin150°
=-√2/2-(-√2/2)-2×1/2
=-1

cos(50°+70°)=cos50°cos70°-sin50°sin70°
-1/2=sin70°cos50°-sin50°sin70°
∴sin50°sin70°=a+1/2

sin315=sin-45=-sin45
-cos135=cos45
sin570=sin210=-sin30
则
sin315-cos135+2sin570的值是-1/2
2.sin20cos50=cos70cos50=a
cos120=cos70cos50-sin70sin50=-1/2
sin70sin50=a+1/2

sin315-cos135+2sin570
=sin(360-45)-cos(180-45)+2sin(540+30)
=sin(-45)+cos(-45)-2sin30
=-sin45+cos45-2sin30
=-√2/2+√2/2-2x1/2
=0-1
=-1
sin50sin70=cos20sin50
于是
cos20sin50-sin20cos50
=sin（50-20）
=sin30
=1/2
所以
sin50sin70=a+1/2