arcsin定义域是[-1,1]
所以-1<=2x-1<=1
0<=2x<=2
0<=x<=1
根号下大于等于0
sinπx>=0
2kπ<=πx<=2kπ+π
2k<=x<=2k+1
要满足0<=x<=1
则k=0
此时是0<=x<=1
所以定义域[0,1]