Помогите решить 4 и 5 задания.

4) =(1-sin^2 (a/8) -(cos^2a +sin^2 a)) /(4sin^4 (a/16))=
=(1-sin^2 (a/8) -1) /(4*sin^4 (a/16))=
=(  -( 2sin(a/16) cos(a/16))^2 ) /(4sin^4 (a/16))=
=(-4sin^2 (a/16) cos^2 (a/16)) /(4sin^4 (a/16))=
=-cos^2 (a/16) /sin^2 (a/16)=-ctg^2 (a/16)
5)tg60=3
 cos15=cos(45-30)=cos45 cos30+sin45 sin30=(2 /2)*(3/2) +2 /2)*(1/2)=
=6/4+2/4;
sin15=sin(45-30=sin45cos30-sin30cos45=2/2 * 3/2 -1/2 *2/2=6/4-2/4
тогда получим
(3*(6/4+2/4) - (6/4-2/4))*72=18/4+6/4-6/4+2/4)*72=
=(32 /4+2/4)*72=42/4 *72=2*72=2*7=14