Question

sin(π/3-a)=1/3，则cos（π/3+2a）

Answer 1

cos(π/3＋2a)=－cos(2π/3－2a)=－cos[2(π/3－a)]=－[1－2sin²(π/3－a)]=－[1－2/9]=－7/9

Answer 2

sin(π/3-a)=cos[(π/3-a)-π/2]=cos(-a-π/6)=cos(π/6+a)=1/3
cos(π/3+2a)=2cos²(π/6+a)-1=-7/9