函数f(x)对一切实数x,y均有f(x+y)-f(y)=(x+2y+1)x成立,且f(1)=0.
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(1)求f(0);
(2)求f(x);
(3)不等式f(x)>ax-5当0<x<2时恒成立,求a的取值范围.