12 multiple-choice questions, progressively harder.
A figure has finite rotational order 14 about O, counting identity. Must it have point symmetry about O?
Solution
Correct answer: D
The order is even, so half of the 14 equally spaced steps reach half of a full turn.
14÷2=714\div2=714÷2=7
That matching 180∘180^\circ180∘ rotation gives point symmetry.
A figure has finite rotational order 15 about O. Which candidate angle must match?
Correct answer: B
Its least turn is found from the full turn.
360∘÷15=24∘360^\circ\div15=24^\circ360∘÷15=24∘
The candidate 72∘72^\circ72∘ is three least turns. None of the other listed angles is an integer multiple of 24∘24^\circ24∘.
A report says a figure has finite rotational order and least positive matching angle 100∘100^\circ100∘. Which conclusion follows?
Correct answer: A
A least matching angle must divide a full turn exactly.
360÷100=3.6360\div100=3.6360÷100=3.6
Order counts distinct rotations and must be an integer. Rounding would change the claimed match.
A circle has a single marked point on its boundary, and the mark is part of the figure. Which statement about rotations around the circle center is true?
Correct answer: C
The unmarked circle matches any center rotation, but a nonidentity turn moves the unique mark to an unmarked position. The whole decorated figure therefore has order 1.
The boundary of a square is symmetric under a half-turn. A short segment is then added from its center to the midpoint of its top side only. Does the new figure keep point symmetry about the center?
A symmetry must include the added segment. Its image extends from the center toward the bottom side, where the figure contains no such segment. Matching the outer boundary is insufficient.
For a finite-order figure, matching angles include 60∘60^\circ60∘ and 180∘180^\circ180∘. No turn strictly between 0∘0^\circ0∘ and 60∘60^\circ60∘ matches. What is its order and point-symmetry status?
The least positive angle is 60∘60^\circ60∘.
360÷60=6360\div60=6360÷60=6
The given matching half-turn also directly establishes point symmetry.
A triangular outline joins (0,0)(0,0)(0,0), (6,0)(6,0)(6,0), and (0,2)(0,2)(0,2). A half-turn about (3,0)(3,0)(3,0) swaps the first two vertices. Is that enough to establish point symmetry?
The point (0,2) is three units left and two above the center, so its image is (6,-2). That point lies below the outline. The first two matches do not rescue the failed third one.
An exact figure has finite order 9 about O, including identity. Which statement explains why it has no point symmetry about O?
The least angle is 40∘40^\circ40∘.
180÷40=4.5180\div40=4.5180÷40=4.5
All matching angles are integer multiples of the least angle. A half-turn is not among them.
A candidate line divides a figure into equal areas. What extra fact would establish that it is a line of symmetry?
Area equality does not describe where the points or boundaries go. The required additional fact is exact whole-figure matching under reflection in that line.
For a circle centered at O with positive radius, why does the finite-order least-angle formula fail?
Every rotation about O preserves the circle, including arbitrarily small positive angles. There is no finite order to put into the formula and no least positive angle to find.
A square boundary has letter labels at its corners used only as names. A quarter-turn moves each label name to a different named corner. Does that alone reject the square symmetry?
The task explicitly excludes the labels from the figure. A symmetry may permute named corners while matching the boundary. If actual printed decorations were included, they too would need to match.
A zero translation and a full-turn rotation are both recorded as symmetries of the same figure. How many distinct plane mappings do these two records describe?
Both send every point to itself. They are two descriptions of the same identity mapping, so they contribute only one distinct symmetry.
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