Geometry, Lesson 1
A Diagram Is Not a Proof
Drag the picture. Break a claim. Discover what refuses to change.
Drag test: what survives?
Dynamic constructionChoose a claim, then move a free point and try to break it.
Free objects You can move A, B, and C.Dependent objects Sides, angles, and measurements update automatically.
A0°
B0°
C0°
sum180°
1. Choose a claim to test2. Drag a point. A claim that says “always” must survive every valid drag.
Practice
Test claims with diagrams
Answer all five questions before the evidence and explanations appear.
Can the three angles make a line?
Press the button and watch the angle pieces meet.
A46.4°
C87.2°
B46.4°
180°
Three changing angles. One straight angle.
Practice
Reason about the angle sum
Finish the set before viewing the complete geometric explanation.
How strong is the evidence?
Tap a kind of evidence and watch what it can do.
Observation
It suggests a claim.
Practice
Judge the strength of evidence
Commit to all five answers, then compare observation, experiment, counterexample, and proof.
Next: make change move.
Analysis turns a journey into a living graph.