f（x）=x|x-a|在[3，+∞）上递增，则a∈______．
当x-a≥0时，f（x）=x（x-a）
f（x）=x（x-a）图象开口向上，对称轴为a 2
函数在[
，+∞）上递增a 2
若f（x）=x|x-a|在[3，+∞）上递增，则a满足
≤3a 2 3-a≥0
即a≤3时，f（x）=x|x-a|在[3，+∞）上递增
当x-a≤0时
f（x）=x（a-x）
图象开口向下，无法保证f（x）在[3，+∞）上递增
故答案为：（-∞，3]
Hello,everyone!__ __John Green. I’m thirteen years old .__ _ is my sister, Kate. She is nine years old. We are___ England. ___ now we are in China. My father is an English____ at a school. My mother is a doctor. __ _works in a hospital. They work very hard(努力). At___I ____ a good friend. His name is Lin Tao. He is ____too. He is from Shanghai. He is nice to me and I ____him.
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