Un2=2I/3,1: Un1=62: (1 + 1/2 + 1/3)Un2 - Un1/2 - Un3/3=0 11Un2 - 2Un3=18 ..................(1)3: (1/3 + 1/4)Un3 - Un1/4 - Un2/3=-I/3 7Un3 + 2Un2=18 ....................(2)解方程就可以。