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二元二次方程Ax~2+Bxy+Cy~2+Dx+Ey+F=0代表一条抛物线的充要条件为：如果令H=A+C,则△,G,H都是在坐标轴的平移和旋转变换之下的不变量
The binary quadratic equation Ax~2+Bxy+Cy~2+Dx+Ey+F=0 represents a necessary and sufficient condition of a parabola: if H=A+C, then △, G, and H are all in the coordinate axis. Translation and rotation transformations