8,(10%)請找出函數f(x，y) = x4 + y4- 4xy + 1的區域極大値及區域最小値以及鞍點 (saddle point)

(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.)