sin (2/3pi-a) - cos(pi+a)

sin(2π/3 -α) -cos(π +α) = sin( π -(π/3 +α))  +cosα = sin(π/3 +α)  +cosα = 
sin(π/3) *cosα + cos(π/3)*sinα + cosα =(√3/2) cosα +(1/2)sinα + cosα =
=(1/2)sinα + (1+√3/2 )cosα .

* * * 
sin(3π/2 -α) - cos(π +α) = -cosα -(-cosα) =  -cosα +cosα =0 .

Смотрите внимательно !