(A 2+B 2) 2+(C 2+D 2) 2 = 2 There are 2 on both sides ((AB) 2+(CD) 2) {(AB) 2+(CD) 2} Let a = A 2. In the same way, you can get B. In the same way, a = b = C=D because a 4+b 4+c 4+d 4 is greater than or equal to 0, so 4abcd is also greater than or equal to 0. If A and B are different symbols, then responsibilities C and D are also different symbols (caused by different formulas added on the left and right sides). If a and c are different symbols, then responsibilities b and d are also different symbols.