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Cho a, b, c đôi một phân biệt. CMR: $\frac{b-c}{(a-b)(a-c)}$ + $\frac{c-a}{(b-c)(b-a)}$ + $\frac{a-b}{(c-a)(c-b)}$ = 2 × ( $\frac{1}{a-b}$ + $\frac{1}{b-c}$ + $\frac{1}{c-a}$ ) Làm ơn..
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