1. Year 9 Geometrical Reasoning
Alternate Angles

2. Here is an angle formed by two lines.
Here is another angle formed by two lines.
We can slide these together to form a ‘zig-zag’.

3. We can slide these together to form a ‘zig-zag’.

4. The RED line is called the TRANSVERSAL because it crosses the other two lines..
Now see how all of the lines can be extended.
Note the acute angles contained in the zig-zag.
We can slide these together to form a ‘zig-zag’.

5. We call these
ALTERNATE ANGLES because they are formed by the transversal and the top line and the transversal and the bottom line. However, they are on ALTERNATE (opposite) sides of the transversal.

6. Do you think that these
ALTERNATE ANGLES will be EQUAL?

7. Watch this copy of the lower angle to see…

11. These
ALTERNATE ANGLES are clearly NOT EQUAL.