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))*7√2=√18/4+√6/4-√6/4+√2/4)*7√2=
=(3√2 /4+√2/4)*7√2=4√2/4 *7√2=√2*7√2=2*7=14