2/3+3/4=8/12+9/12=17/12
1/2+1 1/3=3/6+1 2/6=1 5/6
17/12:1 5/6=17/12*6/11=17/22
√18(cos²7π/8-sin²7π/8)=√18cos7π/4=√18cos(2π-π/4)=√18cosπ/4=
=√18*√2/2=√36/2=6/2=3
<span><span>ctg^2a-tg^2a=cos^2a/sin^2a-sin^2a/cos^2a=(cos^4a-sin^4a)/cos^2a*sin^2a=cos2a/(sinacosa)^2
(cos^2a-sin^2a)/(cos2a)/(sinacosa)^2=sin^2a*cos^2a</span></span>