题目

Besides containing attractive flowers,trees and other plants that beautify the community,eco-friendly rain gardens are healthy for the environment and the people living and working nearby. A rain garden is not very different from a traditional garden.It is just a far more eco-friendly garden.Usually it is built lower than the ground.Rain gardens make smart use of rain and storm water by temporarily holding water from rain and storms and letting it soak(浸入)slowly into the ground before it runs into streams or enters the public drinking water supply. Thus,a rain garden keeps the wat（本题满分6分）先化简，再求值:，其中.