Sinx-cos3x=0
cos(π/2-х) -cos3x=0
-2sin(π/2-х+3х) /2sin(π/2-х-3х) /2=0
-2sin(π/2+2х) /2sin(π/2-4х) /2=0
-2sin(π/4+х) sin(π/4-2х) =0
sin(π/4+х) =0 либо sin(π/4-2х) =0
π/4+х=πk либо π/4-2х=πk
х₁= πk-π/4 либо -2х=πk-π/4
х₁= πk-π/4 либо х₂=-πk/2+π/8= π/8-πk/2