已知5x-4y=0,切xy≠0,
x/y=4/5;
求(x平方-y平方)分之(x平方+y平方)-(x-y)分之(x+y)的值
=(x²+y²)/(x²-y²)-(x+y)²/(x²-y²)
=(x²+y²-x²-y²-2xy)/(x²-y²)
=-2xy/(x²-y²)
=-2x/y/(x²/y²-1)
=-2×(4/5)/(16/25-1)
=-8/5÷(-9/25)
=40/9;
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