giải các phương trình sau

X

xuanquynh97

a) $3x^2+12x-66=0$

\Leftrightarrow $3(x^2+4x+4)-78=0$

\Leftrightarrow $3(x+2)^2=78$

\Leftrightarrow $(x+2)^2=26$

\Leftrightarrow $\left[ \begin{array}{ll} x=\sqrt{26}-2 &\\
x=-\sqrt{26}-2 &
\end{array} \right.$
 
X

xuanquynh97

b) $9x^2-30x+225=0$

\Leftrightarrow $(9x^2-2.3x.5+25)+200=0$

\Leftrightarrow $(3x-5)^2+200=0$

\Rightarrow PT vô nghiệm
 
X

xuanquynh97

c) $x^2+3x-10=0$

\Leftrightarrow $x^2-2x+5x-10=0$

\Leftrightarrow $(x-2)x+5(x-2)=0$

\Leftrightarrow $(x-2)(x+5)=0$

\Leftrightarrow $x=2$ và $x=-5$
 
X

xuanquynh97

d) $3x^2-7x+1=0$

\Leftrightarrow $3(x^2-\dfrac{7}{3}x+\dfrac{49}{36})-\dfrac{37}{12}=0$

\Leftrightarrow $3(x-\dfrac{7}{6})^2=\dfrac{37}{12}$

\Leftrightarrow $(x-\dfrac{7}{6})^2=\dfrac{37}{36}$

\Leftrightarrow $\left[ \begin{array}{ll} x=\dfrac{7+\sqrt{37}}{6} &\\
x=\dfrac{7-\sqrt{37}}{6} &
\end{array} \right.$
 
X

xuanquynh97

e) $x^2-10x-2000=0$

\Leftrightarrow $x^2-50x+40x-2000=0$

\Leftrightarrow $x(x-50)+40(x-50)=0$

\Leftrightarrow $(x+40)(x-50)=0$

\Leftrightarrow $x=-40$ và $x=50$
 
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