1
cos(2π+π/2+6x)cos3x=sinxcos3x
cos(π/2+6x)cos3x=sinxcos3x
-sin6xcos3x=sinxcos3x
sinxcos3x+sin6xcos3x)=0
cos3x(sinx+sin6x)=0
cos3x*2sin3,5x*cos2,5x=0
cos3x=0⇒3x=π/2+πk,k∈z⇒x=π/6+πk/3,k∈z
sin3,5x=0⇒3,5x=πk,k∈z⇒x=2πk/7,k∈z
cos2,5x=0⇒2,5x=π/2+πk,k∈z⇒x=π/5+2πk/5,k∈z
2
cos3x-sin(7x-π/2)=cos5x
cos3x+sin(π/2-7x)=cos5x
cos3x+cos7x=cos5x
2cos5xcos2x-cos5x=0
cos5x(2cos2x-1)=0
cos5x=0⇒5x=π/2+πk,k∈z⇒x=π/10+πk/5,k∈z
2cos2x-1=0⇒cos2x=1/2⇒
2x=-π/3+2πk,k∈z U 2x=π/3+2πk,k∈z
x=-π/6+πk,k∈z U x=π/6+πk,k∈z