Didn't assume, because
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送交者: chouqilozi 于 2009-03-12, 14:58:15:
回答: 最后两行假设了对称性。 可是你凭什么可以假设对称？ 由 HunHunSheng 于 2009-03-12, 14:55:35:
x-------x===o is 2a; and
o===x-------x is 2a, we know the "===" parts are of the same length.
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