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đặt câu hỏi
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toan to hop
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1. Cho cac chu so $1,2,3,4,5,6$, co bao nhieu so tu nhien gom ba chu so khac nhau va nho hon $436 $ 2. Voi cac chu so $1,2,3,4,5,6,7,8,9$ co the lap duoc bao nhieu so chan co ba chu so khac nhau va khong lon hon $789$ |
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sửa đổi
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ai giúp em tí đê cần gấp !!!!
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minh lam cach nay ban xem thu xem sao:$x^{2}$-3x+k-1=0$\triangle $=9-4(k-1)=13-4kpt co nghiem$\Leftrightarrow$ $\triangle$$\geq$0 $\Leftrightarrow$k$\leq$$\frac{13}{4}$Ta co :$ \left\{ \begin{array}{l} x_{1}+x_{2}=3 (1)\\ x_{1} .x_{2}=k-1 (2)\end{array} \right.$ket hop voi yeu cau bai ra , giai he$\Rightarrow$ka, theo bai ra ta co 2x1-5x2=-8 ket hop voi (1) ta duoc he:$\left\{ \begin{array}{l} x_{1}+x_{2}=3\\ 2x_{1}-5x_{2} =-8\end{array} \right.$$\Leftrightarrow$$\left\{ \begin{array}{l} x_{1}=1\\ x_{2}=2 \end{array} \right.$thay vao (2) ta duoc k=3b.theo bai ra ta co: $x^{2}_{1}$-$x^{2}_{2}$=-8 ket hop voi (1)ta co he:$\left\{ \begin{array}{l} (x_{1}-x_{2})(x_{1}+x_{2})=-8\\ x_{1}+x_{2}=3 \end{array} \right.$$\Leftrightarrow$$\left\{ \begin{array}{l} x_{1}=1/6\\ x_{2} =17/6\end{array} \right.$ thay vao (2)$\Rightarrow$k=53/36cau c lam tuong tu
minh lam cach nay ban xem thu xem sao:$x^{2}$-3x+k-1=0$\triangle $=9-4(k-1)=13-4kpt co nghiem$\Leftrightarrow$ $\triangle$$\geq$0 $\Leftrightarrow$k$\leq$$\frac{13}{4}$Ta co :$ \left\{ \begin{array}{l} x_{1}+x_{2}=3 (1)\\ x_{1} .x_{2}=k-1 (2)\end{array} \right.$ket hop voi yeu cau bai ra , giai he$\Rightarrow$ka, theo bai ra ta co 2x1-5x2=-8 ket hop voi (1) ta duoc he:$\left\{ \begin{array}{l} x_{1}+x_{2}=3\\ 2x_{1}-5x_{2} =-8\end{array} \right.$$\Leftrightarrow$$\left\{ \begin{array}{l} x_{1}=1\\ x_{2}=2 \end{array} \right.$thay vao (2) ta duoc k=3b.theo bai ra ta co: $x^{2}_{1}$-$x^{2}_{2}$=-8 ket hop voi (1)ta co he:$\left\{ \begin{array}{l} (x_{1}-x_{2})(x_{1}+x_{2})=15\\ x_{1}+x_{2}=3 \end{array} \right.$$\Leftrightarrow$$\left\{ \begin{array}{l} x_{1}=4\\ x_{2} =-1\end{array} \right.$ thay vao (2)$\Rightarrow$k=-3cau c lam tuong tu
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