1, the first yellow line is, on the left, first write the square as the product of two integrals, and then change one of the integral variables into letters (the integral variable of definite integral can be changed at will); 2. The second yellow line is to change a repeated integral into a double integral. If you don't understand, turn the double integral on the rectangular area into a repeated integral. 3, the second yellow line to the first blue line, this is the average inequality (middle school knowledge); 4. From the first blue line to the second blue line, rotational symmetry is used here. Let me explain below. We notice that this integral area is a square. If you exchange X and Y in this area, you will find that there is no change in this area, which means that X and Y are completely equal in this area, so there are: ∫∫ [f (x)] 2ddxdy = ∫∫ [f (y)].