Grade 8 · Math

8th grade Can three lengths make a triangle? worksheet

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Can three lengths make a triangle?

Grade 8 Math Worksheet

NameDate

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

1.
Bo has three pieces of yarn: 12 cm, 7.5 cm and 5.5 cm. Can they make a triangle?
- No: a triangle needs three equal sides to join up
- No: 12 cm is longer than 5.5 cm
- Yes: 5.5 + 7.5 = 13 cm, longer than 12 cm
2.
Esme has three paper strips: 2.6 cm, 7.5 cm and 4.9 cm. Can they make a triangle?
- Yes: any three paper strips make a triangle
- No: 2.6 + 4.9 = 7.5 cm exactly, so they lie flat
- Yes: 7.5 + 2.6 = 10.1 cm, longer than 4.9 cm
- Yes: 2.6 + 4.9 is not shorter than 7.5 cm
3.
Jun has three crayons: 5.8 cm, 9.5 cm and 3.7 cm. Can they make a triangle?
- No: 3.7 + 5.8 = 9.5 cm exactly, so they lie flat
- Yes: 3.7 + 5.8 is not shorter than 9.5 cm
- Yes: 9.5 + 3.7 = 13.2 cm, longer than 5.8 cm
- Yes: any three crayons make a triangle
4.
Kira has three pencils: 8 cm, 3.2 cm and 4.5 cm. Can they make a triangle?
- Yes: 8 + 3.2 = 11.2 cm, longer than 4.5 cm
- Yes: any three pencils make a triangle
- No: they lie flat, with nothing left over
- No: 3.2 + 4.5 = 7.7 cm, shorter than 8 cm
Noah has two paper strips, 4.6 cm and 9.5 cm, for two sides of a triangle. The third side is more than ? cm and less than ? cm. Type both numbers, smaller first.
Pia has two paper strips, 2.5 cm and 10 cm, for two sides of a triangle. The third side is more than ? cm and less than ? cm. Type both numbers, smaller first.
7.
Ren has 4.5 cm and 7.2 cm paper strips for two sides of a triangle. Which length can the third side be?
- 6.3 cm
- 2.1 cm
- 12.4 cm
- 11.7 cm
8.
Tala has 3.6 cm and 8.4 cm pieces of yarn for two sides of a triangle. Which length can the third side be?
- 12.5 cm
- 3.9 cm
- 10.5 cm
- 4.8 cm
9.
Zee has 2.8 cm and 6.5 cm paper strips for two sides of a triangle. Which length can the third side be?
- 3.5 cm
- 9.8 cm
- 4 cm
- 9.3 cm
Esme has paper strips of 2.2 cm and 9.5 cm for two sides of a triangle. The third side is a whole number of cm. What is the longest it can be?

8th grade Can three lengths make a triangle? worksheet – printable — Print: 1 2 3 4 5 6

About this worksheet

Can three lengths make a triangle?. Every version has 10 questions, and the answer key prints on the second page.

What your child will learn

I can tell whether three lengths make a triangle: the two shorter sides together must be longer than the longest. If they are exactly as long, the shape lies flat.
I can give the range of a third side: more than the difference of the other two, less than their sum.
I can use that range for whole-number sides, isosceles triangles, sides written with x, and the perimeter.

Standard: state-only (related 7.G.A.2)

Words to know

longest side: The side you compare with: the base in the sketch.

flat: The two sides lie along the base with no corner on top: not a triangle.

difference: The bigger side take away the smaller one.

range: All the lengths between the two ends, without the ends themselves.

isosceles triangle: A triangle with two equal sides.

Common mistakes

Your child counted a flat shape as a triangle: when the two shorter sides add to exactly the longest, they lie flat along it.

Your child thought any three lengths make a triangle. Lay two short strips along a long one: if they do not reach past it, there is no triangle.

Your child added the longest side to a short one. That sum is always longer than the other side, so it tells nothing. Add the two shorter sides.

What to say

Ask: is there a corner at the top?

Questions parents ask

What is the most common mistake with can three lengths make a triangle??

Your child counted a flat shape as a triangle: when the two shorter sides add to exactly the longest, they lie flat along it.

How can I help my child with can three lengths make a triangle??

Ask: is there a corner at the top?

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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