Question

1-2cos²a/sin a*cos a

Answer 1

[(sina-cosa)²-1]/[sin²a-cos²a-1]=(sin²a-2sinacosa+cos²a-1)/(sin²a-cos²a-sin²a-cos²a)=
=(1-2sinacosa-1)/-2cos²a=2sinacosa/2cos²a=sina/cosa=tga