This topic means that in the case of decreasing investment every year, it is required that the investment within three years should be equal to the total tourism income. Therefore, if the average growth rate is x, the income in the second year is: 400 * (1+x); The income in the third year is: 400 * (1+x) * (1+x); According to the meaning of the question:

400+400 *( 1+X)+400 *( 1+X)*( 1+X)= 800+800 *( 1- 1/3)+800 *( 1- 1/3)*( 1- 1/2)

Simplify:

X^2+3X- 1=0

Solution:

x =(-3+√ 13)/2； Or X=(-3-√ 13)/2 (negative numbers are omitted here).

Therefore, the average annual growth rate of tourism revenue should be: (-3+√ 13)/2.