If you have just meet a friend from China and you want to keep in touch with him/her in the future, what should you do?
You can ask him/her if you can add him/her on WeChat (a popular social media in China) by asking, "我wo可ke以yi加jia你ni微wei信xin吗ma?"
find the mi/max value for f(x)=x^2-4x
when x=0 or x=4, f(x)=0, since the graph is symmetric, when x=2, f(x) reaches its min value. Thus, f(x)min=2^2-4*2=4-8=-4
X^2-4x=0, find x
x^2-4x=x(x-4)=0, x=0 or x=4