2.6
対称性
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Q1: What does it mean for an ellipse to have symmetry across the x-axis?
Symmetry across the x-axis occurs when the ellipse reflects top to bottom, forming equal halves above and below the horizontal axis. Algebraically, this symmetry is confirmed when replacing y with −y in the ellipse equation yields an equivalent expression, proving the graph remains unchanged under this transformation.
Q2: How can you verify that an ellipse is symmetric about the y-axis?
An ellipse exhibits y-axis symmetry when it reflects left to right across the vertical axis, maintaining equal halves on both sides. You can verify this symmetry by replacing x with −x in the ellipse equation; if the equation remains unchanged, the symmetry is confirmed.
Q3: What is rotational symmetry about the origin in an ellipse?
Rotational symmetry about the origin, called C2 rotational symmetry, occurs when a 180-degree rotation leaves the ellipse unchanged in form. This symmetry is confirmed algebraically when both x and y are replaced with their negatives simultaneously, and the equation still holds true.
Q4: Why does an ellipse centered at the origin have three types of symmetry?
An ellipse centered at the origin demonstrates three symmetries—across the x-axis, across the y-axis, and about the origin—because of its balanced geometric structure. These symmetries reflect the ellipse's underlying mathematical consistency, where distances from two fixed points called foci maintain a constant sum for every point on the curve.
Q5: How does the orientation of an ellipse affect its symmetry properties?
An ellipse with horizontal orientation, stretching farther along the x-axis than the y-axis, maintains all three symmetry types regardless of its orientation. The symmetry properties depend on the ellipse being centered at the origin and the algebraic form of its equation, not on whether it extends more horizontally or vertically.
Q6: What role do foci play in defining an ellipse's symmetric properties?
The foci are two fixed points such that every point on the ellipse maintains a constant sum of distances from them. This defining property creates the ellipse's balanced, symmetric structure, ensuring that the curve exhibits symmetry across both axes and about the origin simultaneously.
Q7: How can algebraic transformations confirm the symmetries of an ellipse on a coordinate plane?
Algebraic transformations confirm ellipse symmetries by testing whether the equation remains unchanged under specific variable replacements. When graphing equations two variables, replacing y with −y tests x-axis symmetry, replacing x with −x tests y-axis symmetry, and replacing both tests origin symmetry.