Grade 5 › G5U09 Geometry: Symmetry

GRADE 5 · ALBERTAOUTCOME AB-G5-5G1.1/5G1.2

L3 · Symmetry in Shapes and Design: Clinic

Learning goal

“I can use the regular-polygon rule (equal sides = lines of symmetry = order of symmetry), I know a circle has infinitely many lines of symmetry and infinite order, I can compare a shape's symmetry counts with its equal sides and angles, and I can classify shapes by their symmetry and recognize symmetry in nature and in First Nations, Métis and Inuit designs.”

Regular polygon — a polygon with all sides equal and all angles equal (a regular pentagon has 5 of each).
Infinite — without end; a circle's symmetry counts are infinite because a matching fold or turn exists at every possible angle.
Classify — to sort shapes into groups by a chosen property - here, by their numbers of lines and order of symmetry.
Plane of symmetry — (from L1) the flat sheet that cuts a 3-D shape into two mirror-image halves.
Worked example

A regular pentagon: 5 equal sides, 5 equal angles. Step 1: Fold test. A line from each vertex to the midpoint of the opposite side makes the halves match - that is one working fold per vertex: 5 lines of symmetry. Step 2: Turn test. The smallest turn moves each vertex to the next vertex: 360 / 5 = 72°, and the shape overlaps 5 times in a full turn - order 5. Step 3: Notice the pattern. Regular hexagon (6 equal sides): 6 lines, order 6. Regular octagon (8): 8 lines, order 8. For a regular polygon, the number of lines of symmetry and the order of symmetry both equal the number of sides - and the number of equal angles is that same number too. One count gives you all three. Step 4: The rule needs the word REGULAR. An isosceles triangle has 2 equal sides but only 1 line of symmetry and no rotation symmetry - equal sides alone do not set the counts; for non-regular shapes, run the fold and turn tests. Say it plainly: regular polygon -> count the sides once and you know both symmetry numbers. Anything else -> test, don't guess.

Check yourself — explain it

The regular-polygon rule works because…

Paired problem — example still visible

A regular hexagon tile: state its lines of symmetry and its order of symmetry. -> 6 lines; order 6.

Faded completion — last step is yours

"A regular polygon has 9 equal sides. Its lines of symmetry number ___, and its order of symmetry is ___." -> 9; 9

Ready to practise?

10 scored questions with a 3-rung hint ladder. Mastery needs 8/10 plus both gate questions.