5)cos²x-sin²x=cos2x=0⇒2x=π/2+πn⇒x=π/4+πn/2 Б
6)(ctg²x+1-1)√(4-x²)=0
ctgx=0⇒x=π/2
x²=4⇒x=-2 U x=2
3корня
7)sin4x/3>-√3/2
-π/3+2πn<4x/3<4π/3 +2πn<br>-π/4+3πn/2x∈(-π/4+3πn/2;π+3π/2)
8)(cosx-sinx)²=cos²x-2cosxsinx+sin²x-1-2cosxsinx=1-2*1/4=1-1/2=1/2
cosx-sinx=√2/2 или cosx-sinx=-√2/2-не удов усл,а-4четв
9)сos7*cos14*cos28*cos56=sin14*cos14*cos28*cos56/2sin7=sin28*cos28*cos56/4sin7=
=sin26*cos56/8sin7=sin112/16sin7
sin115/16sin7