Тригонометрия 10 клас, помогите решить

Всё намнооооооооого проще

1/2(1+cos(2π/5)+1-cos(4π/5)+1+cos(6π/5)+1-cos(8π/5))==2+1/2(cos(2π/5)+cos(6π/5))-1/2(cos(4π/5)+cos(8π/5))==2+1/2*2cos(4π/5)cos(π/5)-1/2*cos(6π/5)cos(2π/5)==2+cos(2π/5)(cos(4π/5)-cos(6π/5))=2-2cos(2π/5)sin(π/5)sinπ=2,т.к.sinπ=0