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[tex]\displaystyle\bf\\\frac{3x+y}{(x+y)^{2} } +\frac{x+3y}{(x+y)^{2} } =\frac{3x+y+x+3y}{(x+y)^{2} } =\\\\\\=\frac{4x+4y}{(x+y)^{2} } =\frac{4\cdot(x+y)}{(x+y)^{2} } =\frac{4}{x+y}[/tex]