Lesson 7 of 14
Finding an angle nobody measured
Show on a big screenA cricket pitch has parallel creases, cut by return creases. One corner's angle is known.
Step 1 of 4
- Start at a known angle: 52°, upper crossing.
- Straight line: the wedge beside it is 180 − 52 = 128°.
- Corresponding: the matching corner at the lower crossing is 128°.
- Vertically opposite: the wedge facing it is also 128°.
The rule
The angle you know is usually not the angle you want. Walk to the angle you want, one named fact at a time. Each fact says where you may move. Each fact also says what happens to your number.
| Fact | Moves you to | Number becomes |
|---|---|---|
| vertically opposite | across the same crossing | unchanged |
| angles on a line | the wedge beside it | 180° minus it |
| corresponding | same corner, other crossing | unchanged, needs parallel |
| alternate | inside wedge, opposite side | unchanged, needs parallel |
| co-interior | inside wedge, same side | 180° minus it, needs parallel |
The last three need the two lines parallel. Check: 52° and 128° lie side by side on a line. 52 + 128 = 180.
One angle is given. Tap x. Then read the walk of facts.
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Try the lab to finish this lesson.