Ptdt $AB$:$x+2y-5=0$Ta co:$\frac{-1}{-2}=\frac{2}{4}\Rightarrow \Delta _1//\Delta _2$Goi $M=AB\cap \Delta _1$ Toa do cua M la nghiem cua he:$\left\{ \begin{array}{l} x+2y-5=0\\ -x+2y-7=0 \end{array} \right.\Leftrightarrow \left\{ \begin{array}{l} x=-1\\ y=3 \end{array} \right.$$\Rightarrow M(-1;3)$Goi $N=AB\cap \Delta _2$ toa do cua $N$la nghiem cua he:$\left\{ \begin{array}{l} x+2y-5=0\\ -2x+4y-5=0 \end{array} \right.\Leftrightarrow \left\{ \begin{array}{l} x=5/4\\ y=15/8 \end{array} \right.$$\Rightarrow N(5/4;15/8)$$C$ cach deu $\Delta_1 ,\Delta _2 \Rightarrow C$ la trung diem cua $MN:\left\{ \begin{array}{l} x_C=\frac{-1+5/4}{2}\\ y_C=\frac{3+15/4}{2} \end{array} \right.$Vay$ C(\frac{1}{8};\frac{27}{8})$

Ptdt $AB$:$x+2y-5=0$Ta co:$\frac{-1}{-2}=\frac{2}{4}\Rightarrow \Delta _1//\Delta _2$Goi $M=AB\cap \Delta _1$ Toa do cua M la nghiem cua he:$\left\{ \begin{array}{l} x+2y-5=0\\ -x+2y-7=0 \end{array} \right.\Leftrightarrow \left\{ \begin{array}{l} x=-1\\ y=3 \end{array} \right.$$\Rightarrow M(-1;3)$Goi $N=AB\cap \Delta _2$ toa do cua $N$la nghiem cua he:$\left\{ \begin{array}{l} x+2y-5=0\\ -2x+4y-5=0 \end{array} \right.\Leftrightarrow \left\{ \begin{array}{l} x=5/4\\ y=15/8 \end{array} \right.$$\Rightarrow N(5/4;15/8)$$C$ cach deu $\Delta_1 ,\Delta _2 \Rightarrow C$ la trung diem cua $MN:\left\{ \begin{array}{l} x_C=\frac{-1+5/4}{2}\\ y_C=\frac{3+15/8}{2} \end{array} \right.$Vay$ C(\frac{1}{8};\frac{39}{16})$