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 - Challenge Me

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

The angles of Kira's octagon shape drawn on paper are x°, x°, x°, x°, (x + 30)°, (x + 30)°, (x + 30)° and (x + 30)°. How many degrees is the largest angle?
The angles of Luz's pentagon shape cut from paper are (x + 50)°, (x + 50)°, (x + 10)°, (x + 10)° and x°. How many degrees is the largest angle?
The inside angles of Milo's paper polygon add up to 1440°. How many sides does it have?
a polygon with its sides not counted; the angle total 1980° written inside. The inside angles of Noah's paper cut-out with many sides add up to 1980°. How many sides does it have?
The inside angles of Pia's polygon drawn in pencil add up to 2340°. How many sides does it have?
The inside angles of Ren's big polygon drawn in crayon add up to 3240°. How many sides does it have?
Each inside angle of Tala's regular shape drawn on paper is 140°. How many sides does it have?
Each inside angle of Zee's regular paper cut-out is 150°. How many sides does it have?
Each inside angle of Ari's regular paper polygon is 160°. How many sides does it have?
a regular polygon, each inside angle 156°; one side extended to show the outside angle. Each inside angle of Bo's regular shape cut from paper is 156°. How many sides does it have?

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

See all grade 8 math

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