数学试题讲解

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下列运算正确的是

\frac{y}{- x - y} = - \frac{y}{x - y}

\frac{2 x + y}{3 x + y} = \frac{2}{3}

\frac{x^{2} + y^{2}}{x + y} = x + y

\frac{y + x}{x^{2} - y^{2}} = \frac{1}{x - y}