I did not use squre brackets in my expression,the squre brackets are only  in the result

maybe I didnot express clearly

what I want to do is in the fileView 13857_2009.12.12-new.mw on MapleNet or Download 13857_2009.12.12-new.mw
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But the last step may be wrong, how should I correct it?

thank you very much!

Thank you  PatrickT,the code I gave is the result of collect(L_a,bb), after collet I want to eliminate to get an expression.So I want to get a more simply result derictly,what should I do?

Thank you