1) sin(3π/2+a)=-cos a, tg(π/2+a)=-ctg a, ctg(2π-a)=-ctg a, sin(π+a)= - sin a
-cos a*(-ctg a)/(-ctg a*(- sin a))= cos a/sin a=ctg a
2) cos a*tg a/sin^2 a=cos a*sin a/cos a/sin^2 a=sin a/sin^2 a=1/sin a
ctg a*cos a/sin a=ctg a*ctg a=ctg^2 a
3)(1/sin a+ctg a)(1/sin a+ctg a)=1/sin^2 a - ctg^2 a=(1-cos^2 a)/sin^2 a =
=sin^2 a/sin^2 a=1