Hello,
Since 3sin x-4sin^3x=sin (3x) (**)
3sin x-4sin^3x=-1
==>sin (3x)=-1
==>3x=3π/2 +2kπ
==>x=π/2+2kπ/3, k being an integer
(**)
sin (3x)=sin(x+2x)=sin x cos2x+cos x sin 2x
=sin x(cos²x-sin²x)+2cos²x sin x
=sin x(cos²x+2cos²x-sin²x)
=sinx(3(1-sin²x)-sin²x)
=3sin x-4sin^3x
