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> Prove that the function $f : \mathbb R\_{>0}^4 \to \mathbb R$ defined by $$f(x\_1,x\_2,x\_3,x\_4) = \dfrac{x\_1}{x\_1 x\_3+x\_2 x\_4}$$ is convex.
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I am not sure if this assertion is correct but I do believe it is. Any help is appreciated!