Lesson 3 of 14
The four angles at a crossing
Show on a big screenAn old kite frame has two sticks tied at one crossing. How many angles, or wedges, are there at the knot?
Step 1 of 4
- Two lines cross at one point, the vertex.
- Four wedges open out. Label them a, b, c, d.
- a and c face each other. Mark both with one tick.
- b and d face each other. Mark both with two ticks.
The rule
Two crossing lines make four angles at the vertex. A pair that faces straight across the vertex is called vertically opposite. Vertically opposite angles are always equal.
| Wedge | Faces across | So its size is |
|---|---|---|
| a | c | the same as c |
| b | d | the same as d |
Watch out!
"Vertically opposite" does not mean the wedge on top and below. It means facing straight across the crossing point. Turn the page a quarter turn. Top and bottom become left and right. The pairs stay the same.
Slide angle a. Turn the picture. Which angles match?
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Try the lab to finish this lesson.