sedinalana 24
(ctg²2a-1)/2ctg2a -cos8a*ctg4a=ctg4a-cos8a*ctg4a=
=ctg4a*(1-cos8a)=cos4a/sin4a *2sin²4a=2sin4a8cos4a=sin8a
25
(sin^4a-cos^4a+cos²a)/2(1-cosa)=
=[(sin²a+cos²a)(sin²a-cos²a)+cos²a]/2(1-cosa)=
=(sin²a-cos²a+cos²a)/2(1-cosa)=sin²a/2(1-cosa)=
(1-cos²a)/2(1-cosa)=(1-cosa)(1+cosa)/2(1-cosa)=(1+cosa)/2=
=2cos²(a/2)/2=cos²(a/2)