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`M=(b^2c^2)/a + (c^2a^2)/b+(a^2b^2)/c`
`-> M = (a^2b^2c^2)/a^3 + (a^2b^2c^2)/b^3 + (a^2b^2c^2)/c^3`
`->M = a^2b^2c^2 (1/a^3 + 1/b^3 + 1/c^3)`
`1/a+1/b+1/c=0`
Đặt `1/a=x, 1/b=y, 1/c = z`
`-> x+y+z=0`
`-> (x+y)^3=-z^3`
`->x^3 + y^3 +z^3 - 3xyz =0`
`->x^3 + y^3 +z^3=3xyz`
`-> 1/a^3 + 1/b^3 + 1/c^3 = 3/(abc)`
Do đó : `M = a^2b^2c^2 . 3/(abc)`
`->M =3abc` (đpcm)
