9.設D為心臟線r=2(1-cosθ)所成的區域,且在D上的點(x,y), 其密度函數 p(x,y)=，試求其總質量(total mass) 及質量中心(center of mass)

(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.)