Write an analysis of high school mathematics test questions

Let’s take a look at a high school mathematics test question: If x+y=3, what is the value of x^2+y^2? First of all, according to the information given in the question, we can know that the sum of x and y is 3, so the sum of the squares of x and y can be expressed as: x^2 + y^2 = (x+y)^2 - 2xy = 9 - 2xy. Since the product of x and y is 2xy, the value of x^2+y^2 is 7.