Toán Lớp 8: Cho a + 2b + 3c = 0. Chứng minh : a^3 + 8b^3 + 27c^3 = 18abc
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Toán Lớp 8: Cho a + 2b + 3c = 0. Chứng minh : a^3 + 8b^3 + 27c^3 = 18abc
Comments ( 2 )
a + 2b = -3c
(a + 2b) 3 = (- 3c) 3 (Bằng hình khối)
a3 + (2b) 3 + 3 (a) (2b) (a + 2b) = -27c3
a3 + 8b3 + 6ab (-3c) = -27c3 [a + 2b = -3c]
a3 + 8b3-18abc = -27c3
=>a3 + 8b3 + 27c3 = 18abc