方程X²-Y²=4XY两边同除XY得X/Y-Y/X=4
则(X/Y-Y/X)²=4²
X²/Y²+Y²/X²-2=16
X²/Y²+Y²/X²=18
故(X^4+Y^4)/(XY)²=X²/Y²+Y²/X²=18