13. 假設，試證f(x, y)在點(0,0)處偏導數(partial derivative) 存在，但不可微(differential does not exist)。

(13) A logistic population model with relative growth rate 0.1 per year and carrying capacity 50 thousand can be expressed by the differential equation =0.1P(1 - ), with P in thousands and t in years. Given that the initial population is 9 thousand. Find the population size after 20 years. (If you memorized the formula, then you need to derive it for this problem.)