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tuananhplay

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conga222222

$\eqalign{
& \Delta = {k^2} - 4tz < 0 \cr
& \leftrightarrow 4tz > {k^2} \to z > 0\;\left( {do\;{k^2} \ge 0;t > 0} \right) \cr
& {{t + k + z} \over {k - t}} > 3 \leftrightarrow t + k + z > 3\left( {k - t} \right)\;\left( {do\;k - t > 0} \right) \cr
& \leftrightarrow 4t + z > 2k \cr
& ma\;cosi: \cr
& 4t + z \ge 2\sqrt {4tz} > 2k\;\left( {4tz > {k^2}} \right) \cr
& \to dung\; \to dpcm \cr} $
 
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