Grade 8 · Math

8th grade Can three lengths make a triangle? worksheet

6 printable versionsBuilding Confidence10 questions · answer keyFree · no account

Level: Building Confidence · On Level · Challenge Me

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Can three lengths make a triangle?

Grade 8 Math Worksheet - Building Confidence

NameDate

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

1.
Ari lays out three paper strips: 9 cm, 5 cm and 7 cm. The sketch adds the two shorter ones. Can the three make a triangle?
- No: a triangle needs three equal sides
- Yes: 5 + 7 = 12 cm, longer than 9 cm
- No: 9 cm is longer than 5 cm
2.
Bo lays out three pieces of yarn: 2.5 cm, 5.5 cm and 4 cm. The sketch adds the two shorter ones. Can the three make a triangle?
- No: a triangle needs three equal sides to close up
- No: 5.5 cm is longer than 2.5 cm
- Yes: 2.5 + 4 = 6.5 cm, longer than 5.5 cm
3.
Esme lays out three crayons: 7 cm, 11 cm and 4 cm. The sketch adds the two shorter ones. Can the three make a triangle?
- Yes: 11 + 4 = 15 cm, longer than 7 cm
- Yes: any three crayons make a triangle, whatever the lengths
- Yes: 4 + 7 is not shorter than 11 cm
- No: 4 + 7 = 11 cm exactly, so they lie flat
4.
Jun lays out three paper strips: 8 cm, 3.5 cm and 4.5 cm. The sketch adds the two shorter ones. Can the three make a triangle?
- Yes: any three paper strips make a triangle
- Yes: 3.5 + 4.5 is not shorter than 8 cm
- Yes: 8 + 3.5 = 11.5 cm, longer than 4.5 cm
- No: 3.5 + 4.5 = 8 cm exactly, so they lie flat
Kira hinges two paper strips, 5 cm and 11 cm, as two sides of a triangle. The third side is more than ? cm and less than ? cm. Type both numbers, smaller first.
Luz hinges two pieces of yarn, 6 cm and 13 cm, as two sides of a triangle. The third side is more than ? cm and less than ? cm. Type both numbers, smaller first.
Noah hinges two paper strips, 7 cm and 15 cm, as two sides of a triangle. The third side is more than ? cm and less than ? cm. Type both numbers, smaller first.
8.
Ren has 5 cm and 8 cm crayons for two sides of a triangle. The sketch shows the hinge. Which length can the third side be?
- 7 cm
- 15 cm
- 13 cm
- 2 cm
9.
Tala has 2.5 cm and 6 cm pieces of yarn for two sides of a triangle. The sketch shows the hinge. Which length can the third side be?
- 3.5 cm
- 2 cm
- 9.5 cm
- 5 cm
10.
Zee has 3 cm and 11 cm paper strips for two sides of a triangle. The sketch shows the hinge. Which length can the third side be?
- 12 cm
- 16 cm
- 8 cm
- 6 cm

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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