Rotation means that the position equation remains unchanged, such as x 2+y 2 = 1 to find the maximum xy, and the xy switch becomes y 2+x 2 = 1 to find the maximum yx. It can be seen that the two problems are equivalent, so xy rotates. Note that x 2+4y 2 = 1 is used to find the maximum value of xy, and the xy exchange is changed to y 2+4x 2 = 1 to find the maximum value of yx. The formula has changed, xy is asymmetrical, but xt rotates after 2y = t. When the formulas with rotation are equal (the former x=y and the latter x=2y), there is a maximum value, but I don't know whether it is the maximum value or the minimum value. Generally, it is used to choose the maximum question in the blank, use big questions and test answers. There are three unknowns that can be exchanged in three revolutions. If x 2+y 2+z 2 =1,find the maximum value of x+y+z, then xyz rotates, but xyz in (x+y)(y+z) does not.