L4 · The 90° referent: finding right angles in the environment

Learning goal

“I can test an angle with the corner of a book or a sheet of paper: if it matches exactly, it is a right angle - a right angle measures 90° - and I can find right angles all around me.”

Right angle — an angle that matches the corner of a book or a sheet of paper exactly. A right angle measures 90°.

Referent — a known angle you picture, or carry with you, to compare other angles with.

Degree — a unit used to measure angles. The right angle is 90 degrees, written 90°.

Square corner — a corner that matches a right angle exactly, like the corners of a page or a tile.

Worked example

Take the corner of a sheet of paper. That corner is our referent - a right angle. A right angle measures 90°. Step 1: Test the corner of a book. Lay the paper corner into it: the arms match exactly, edge for edge. Step 2: The book's corner is a right angle too (90°). Step 3: Test a door opened wide. The paper corner fits inside the door's opening with room left over - the door's opening is wider than the referent, so it is NOT a right angle. Say it plainly: exact match = right angle. Wider or narrower than the referent = not a right angle, whatever direction it points.

Check yourself — explain it

An angle is a right angle only if...

Paired problem — example still visible

You test the corner of a floor tile with the paper corner. It matches exactly. What is the tile's corner? -> A right angle (90°)

Faded completion — last step is yours

"A right angle measures ___°. We test for one with the corner of a ___" -> 90; sheet of paper

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10 scored questions with a 3-rung hint ladder. Mastery needs 8/10 plus both gate questions.

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