數一條[要step~]
回答 (1)
2009-10-04 9:51 pm
✔ 最佳答案
(a + b - c)^2 = 7^2a^2 + b^2 + 2ab - 2(a + b)c + c^2 = 49
a^2 + b^2 + c^2 + 2ab - 2(bc + ac) = 49 (Since ab = bc + ac)
a^2 + b^2 + c^2 + 2ab - 2ab = 49
a^2 + b^2 + c^2 = 49
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https://hk.answers.yahoo.com/question/index?qid=20091004000051KK02612