清华大学出版社 TSINGHUA UNIVERSITY PRESS a水(b+c/d)十e*h一g*f(s,t,x十y) a*(b+c/d) e*h-g*fs, t, xty b+c/d h g*fs, t, x+y) c/d fs, t, xty b zty C sa*(b＋c/d)＋e*h－g*f(s，t，x＋y)