I suspect a typo
http://www.wolframalpha.com/input/?i=Cos%5E4(x)%2Bsin%5E4(4)%3Dcos(2x)
notice the graph of the left side does not intersect the graph of the right side
Cos^4(x)+sin^4(4)=cos(2x)
Plz help.
2 answers
I think you misplaced a minus sign. Fix it and think "difference of squares"