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C

conga222222

$\eqalign{
& \left\{ \matrix{
x - {\mathop{\rm siny}\nolimits} = 0 \cr
y - sinz = 0 \cr
z - sinx = 0 \cr} \right. \cr
& \to x + y + z - \sin x - \sin y - \sin z = 0\;\left( 1 \right) \cr
& - 1 \le \sin x = z \le 1 \cr
& - 1 \le \sin y = x \le 1 \cr
& - 1 \le \sin z = y \le 1 \cr
& TH1:\;x = 0 \to y = 0,z = 0 \cr
& TH2:\;x > 0 \to y > 0\;\left( {tu\;cm\;nhe} \right) \to z > 0 \cr
& xet\;f(t) = t - sint \cr
& {f^/}(t) = 1 - \cos t \ge 0 \cr
& \to f(t) > 0\;\forall t > 0 \cr
& \to x - \sin x > 0 \cr
& y - \sin y > 0 \cr
& z - \sin z > 0 \cr
& \to \left( 1 \right)\;vo\;nghiem\;hay\;he\;vo\;nghiem \cr
& tuong\;tu\;voi\;x < 0\;thi\;he\;cung\;vo\;nghiem\;vay\;he\;co\;nghiem\;duy\;nhat\;x = y = z = 0 \cr} $
 
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