y = 2的X次方/(2的X次方+1)
定义域x∈R
y = 2的X次方/(2的X次方+1) = 1/{1+1/(2的X次方)}
0＜2的X次方＜+∞
+∞＞1/(2的X次方)＞0
1＜1+1/(2的X次方)＜+∞
0＜1/{1+1/(2的X次方)}＜1
值域（0,1）