For the Malthus model, use this link http://en.wikipedia.org/wiki/Malthusian_growth_model
For the Logistic model, use this link http://mathworld.wolfram.com/LogisticEquation.html
假设时刻t的人口为x(t)请给出描述人口增长的指数增长模型（Malthus模型）和阻滞增长模型（Logistic模型）
For the Malthus model, use this link http://en.wikipedia.org/wiki/Malthusian_growth_model
For the Logistic model, use this link http://mathworld.wolfram.com/LogisticEquation.html