Пусть arctg5=t; tgt=5
1+tg^2t=1/cos^2t; cos^2t=1/(1+5^2); cos^2t=1/26
sin^2 t+cos^2 t=1; sin^2 t=1-1/26=25/26
sint=√(25/26)=5√26 /26
sin arctg5=5√26 /26
2)cos7x -cos3x=0
-2sin((7x+3x)/2) *sin((7x-3x)/2)=0
-2sin5x sin2x=0
sin5x=0 ili sin2x=0
5x=πn 2x=πn; n-celoe
x=π/5 *n x=π/2 *n
Ответπ/5 *n; π/2 *n; n-celoe