函数方程。答案应该都是y=x/2

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(1)求所有函数f:$\Bbb R^+\to\Bbb R^+$，$\forall x>0$满足f(f(x))=$\frac52f(x)-x$且$\frac x2\leq f(x)<2x$
(2)求所有函数f:$\Bbb R^+\to\Bbb R^+$，$\forall x>0$满足f(f(x))=$\frac52f(x)-x$且$f(x)\leq 2x-\frac x{1+x^{\frac32}}$