1. cos3x=2sin(3π/2-x)
2. cosx-sin11x=o
3. sinx+sin7x-cos5s-cos(π-3x)=o

1cos3x=-2cosx4cos³x-3cosx+2cosx=04cos³x-cosx=0cosx(4cos²x-1)=0cosx=0⇒x=π/2+πn,n∈z4cos²x-1=04(1+cos2x)/2=11+cos2x=1/2cos2x=-1/22x=+-2π/3+2πkx=+-π/3+πk,k∈z2cosx-cos(π/2-11x)=0-2sin(6x-π/4)sin(-5x+π/4)=02sin(6x-π/4)sin(5x-π/4)=0sin(6x-π/4)=06x-π/4=πn6x=π/4+πnx=π/24+πn/6,n∈zsin(5x-π/4)=05x-π/4=πk5x=π/4+πkx=π/20+πk/5,k∈z3(sinx+sin7x)+(cos3x-cos5x)=02sin4xcos3x+2sin4xsinx=02sin4x*(cos3x+sinx)=0sin4x=04x=πnx=πn/4cos3x+cos(π/2-x)=02cos(x+π/4)cos(2x-π/4)=0cos(x+π/4)=0x+π/4=π/2+πkx=π/4+πk,k∈zcos(2x-π/4)=02x-π/4=π/2+πm2x=3π/4+πmx=3π/8+πm/2,m∈z