x^4-XY^3-X^3Y-3X²Y+3XY²+Y^4=X^3(X-Y)-Y^3(X-Y)-3XY(X-Y)=X^3-Y^3-3XY=(X-Y)(X²+XY+Y²)-3XY=X²+XY+Y²-3XY=X²-2XY+Y²=(X-Y)²=1
