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[Drago]

[trieutuyennham]

Câu bất

a) (1)$\Leftrightarrow$ 6/a²+6/b²+6/c²$\leq$1/a+1/b+1/c+1

$\Leftrightarrow$ 2(1/a+1/b+1/c)²-(1/a+1/b+1/c)-1$\leq$0

$\Leftrightarrow$ (1/a+1/b+1/c-1)(2/a+2/b+2/c+1$\leq$)0

$\Leftrightarrow$1/a+1/b+1/c$\leq$1

b)

M$\leq$1/12.(1/a+1/b+1/c)$\leq$1/12