Question

Cos3© is equivalent to

Answer 1

$$Cos(3x)=Cos(2x+x)=\\ =Cos(2x)Cos(x)-Sin(2x)Sin(x)=\\ =(Cos^2(x)-Sin^2(x))Cos(x)-2Sin(x)Cos(x)Sin(x)=\\ =Cos^3(x)-3Sin^2(x)Cos(x)=\\ =Cos^3(x)-3(1-Cos^2(x))Cos(x)=\\ =4Cos^3(x)-3Cos(x)$$