Grade 8 · Math

8th grade Angles of a polygon: add up the triangles worksheet

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Angles of a polygon: add up the triangles

Grade 8 Math Worksheet

NameDate

Circle the best answer, or write your answer on the line.

Esme measures every inside angle of a 7-sided paper shape and adds them up. What should the total be, in degrees?
Jun measures every inside angle of a 9-sided paper cut-out and adds them up. What should the total be, in degrees?
Kira measures every inside angle of a 10-sided shape drawn on paper and adds them up. What should the total be, in degrees?
Luz measures every inside angle of a 12-sided shape cut from paper and adds them up. What should the total be, in degrees?
Milo draws a regular octagon shape drawn on paper (8 equal sides and angles). All its inside angles are equal. How many degrees is each one?
Ren draws a regular 20-sided shape drawn in crayon. All its inside angles are equal. How many degrees is each one?
Tala's hexagon paper cut-out has five angles of 120°, 135°, 112°, 128° and 117°. What is the sixth angle, in degrees?
Zee's paper hexagon has five angles of 95°, 140°, 132°, 118° and 125°. What is the sixth angle, in degrees?
Ari's hexagon shape drawn on paper has five angles of 150°, 102°, 124°, 116° and 131°. What is the sixth angle, in degrees?
Bo's hexagon shape cut from paper has five angles of 108°, 144°, 127°, 99° and 136°. What is the sixth angle, in degrees?

8th grade Angles of a polygon: add up the triangles worksheet – printable — Print: 1 2 3 4 5 6

About this worksheet

Angles of a polygon: add up the triangles. Every version has 10 questions, and the answer key prints on the second page.

What your child will learn

I can split a polygon into triangles from one corner (two fewer than its sides) and find its angle total.
I can find each angle of a regular polygon, and a missing angle of any polygon.
I can solve a polygon's angles written with x, and find the number of sides from the total or from one angle.

Standard: state-only (CCSS 8.G.A.5 names triangles only)

Words to know

polygon: A flat shape with straight sides that closes up.

diagonal: A segment joining two corners that are not next to each other.

interior (inside) angle: The angle inside the shape at a corner.

regular polygon: All sides equal and all angles equal.

outside angle: The turn at a corner when you walk around the edge.

Common mistakes

Your child counted one triangle for every side. Diagonals from one corner make two fewer triangles than sides, because the two sides at that corner make no new triangle.

Your child used one fewer than the number of sides. A polygon split from one corner gives two fewer triangles than sides.

Your child used 360° for each triangle. A triangle's angles add to 180°.

Questions parents ask

What is the most common mistake with angles of a polygon: add up the triangles?

Your child counted one triangle for every side. Diagonals from one corner make two fewer triangles than sides, because the two sides at that corner make no new triangle.

Are these 8th grade math worksheets free to print?

Yes. All 6 printable versions at each level are free to print, with the answer key, and no account is needed.

More grade 8 math

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