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Geometry and Measurement 6-8 / Week 06 / Build Day
6/6
Week 06 · Triangles with Rules, and Slices

Build Day

Build Day: The roof frame
// When three straws make a roof, and what a cut face shows
⏱ about 20 min

Build Day: The Roof Frame

Saturday at the Blueprint Table, Rocket hands out straws. "Build Day. Everyone makes a roof triangle that closes."

"Then we slice," Raven says, setting the clay on the tray. "What do you notice about the cut faces?"

"They match the shapes we learned," Rocket's cousin says. "Flat cut, base shape."

Nova hovers above the straw pile, her light measuring the longest one. "Predict first," she says. "Then push."

"Three, three and eight," the cousin says. "It will not close. Six is less than eight."

Rocket grins. "Try 5, 9, 9. That one always works."

Nova hums. "One roof, four matching triangles."

  1. Three straws make a triangle only when the two shorter ones add to more than the longest.
  2. Three sides that close make exactly one triangle. Three corners adding to 180 make many. Other sets make none.
  3. Two sides with the corner between them, or two corners with the side between them, make one triangle.
  4. A flat cut copies the base: a rectangle for a box, a square for a pyramid.
  5. An upright cut through a box shows a rectangle. A cut through a pyramid's top shows a triangle.
◇ THE ROOF FRAME WITH A GROWN-UP
Cut straws to 5, 9 and 9 cm, four sets. Predict with the rule, then build four triangles.
Tape the four triangles around a 5 cm square of cardboard to make a pyramid roof.
Shape a clay box and a clay pyramid. Slice each flat, then upright, with the string loop. Name every face.
Finish by trying one straw set that should not close. Say why before you push the ends together.
For a grown-up
This week your student learned the triangle rule and when three measures make one, many or no triangles.
Ask them why 3, 4 and 7 do not close. Listen for "equal is not more than".
Scissors with you nearby. Clay is sliced with string, never a blade, and stays on the tray.
Safety first
Cut straws with scissors, with a grown-up nearby. Nobody runs with straws.
Slice clay with the string loop only. Wash hands when you finish.
BUILD DAY CHECK
  • Read the question.
  • Tap your answer.
Can straws 5, 9 and 9 cm long make a triangle?
Can straws 3, 3 and 8 cm long make a triangle?
Three corners: 40°, 40° and 100°. How many different triangles have these corners?
A pyramid is sliced with a flat cut. What shape is the cut face?
Two straws are 9 cm each. The third must be shorter than how many cm? Type the number.
WHY THIS EXERCISE9 + 9 = 18, so any third straw shorter than 18 cm closes the triangle.
Draw your pyramid roof: a 5 cm square with four 5, 9, 9 triangles folded up around it.

Great building, crew. Next week the string goes around the can, and a famous number appears.

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