Sorry for my naive and rather stupid questions. I am more than thankful for all the helps I have received so far from this community. I majored in Economics and had minimal exposure to mathematical training back in college. (Not even Calculus I II III and systematic teaching in linear algebra). Most of my understanding of mathematical concepts were taught by my father over our casual conversation over these years. It didn't occur to me how little thoughts I have put into maths until recent years. It may be too late for me to be good at maths but I hope it is too late to try to understand more.