2sin15° cos15°/cos2 67,5°- sin2 67,5°

2sinxcosx=sin2x cos²y-sin²y=cos2y ---------------------------------- 2sin15cos15/(cos²67,5-sin67,5)=sin30/cos135=sin30/cos(180-45)= =sin30/(-cos45)=1/2:(-√2/2)=1/2*(-√2)=-1/√2