Question 203947
<pre><font size = 4 color = "indigo"><b>

        {{{Cos^2(2y)-4Sin^2(y)Cos^2(y)=Cos(4y)}}}

Work with the right side:
                          {{{Cos(4y)}}} 

                          {{{Cos(2(2y))}}}

                          {{{Cos^2(2y)-Sin^2(2y)}}}

                          {{{Cos^2(2y)-(Sin(2y))^2}}}
               
                          {{{Cos^2(2y)-(2Sin(y)Cos(y))^2}}}
    
                          {{{Cos^2(2y)-2^2Sin^2(y)Cos^2(y)}}} 

                          {{{Cos^2(2y)-4Sin^2(y)Cos^2(y)}}}

Edwin</pre>