1)tg^2a-sin^2 a=sin^2 a /cos^2 a-sin^2 a=sin^2 a ((1-cos^2 a)/cos^2 a)=
=(sin^2 a sin^2 a)/cos^2 a=tg^2 a sin^2 a;
2)ctg^2 a-cos^2 a=cos^2 a/sin^2 a-cos^2 a=cos^2 a (1/sin^2 a-1)=
=cos^2 a((1-sin^2 a)/sin^2 a)=cos^2 a cos^2 a/sin^2 a=ctg^2 acos^2 a;
3)(tg^2 a sin^2 a)/(ctg^2 a cos^2 a)=(tg^2 a *tg^2 a )/ctg^2 a=tg^6 a
4)tg^6 a-(tg^6 a -7)=tg^6 a-tg^6 a +7=7