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）
