Question

sin(六分之派-角a)等于三分之一.求cos(六分派加2阿尔法)

Answer 1

sin(π/6-a)=1/3,
∴cos(π/3-2a)=cos[2(π/6-a)]=1-2[sin(π/6-a)]^2=1-2/9=7/9,
∴sin(π/3-2a)=土4√2/9,
∴cos(π/6-2a)=cos[(π/3-2a)-π/6]
=(7√3土4√2)/18.