给出下列命题：
（1）幂函数的图象都过点（1，1），（0，0）；
（2）幂函数的图象不可能是一条直线；
（3）n=0时，函数y=xn的图象是一条直线；
（4）幂函数y=xn当n＞0时，是增函数；
（5）幂函数y=xn当n＜0时，在第一象限内函数值随x值的增大而减少．其中正确的命题序号为______．
（1）对于y=x-1，其图象不过（0，0），故可排除（1）；
（2）幂函数y=x的图象是一条直线，故可排除（2）；
（3）n=0时，函数y=xn的图象不是一条直线（点（0，1）除外），故可排除（3）；
（4）幂函数y=x2，在其定义域R上不是增函数，故可排除（4）；
（5）幂函数y=xn当n＜0时，在第一象限内函数值随x值的增大而减少，正确．
故答案为：（5）．
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