Similar Triangles in GeoGebra
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Draw a segment 3 units long
Choose the circle with centre and radius tool from the sixth box along the top
Choose point B on the segment and enter a radius of 4 units
Choose the angle with given size tool from the eighth box along the top
Click on point A, then point B and then enter 80° and select clockwise
Draw a ray from point B through A’
Select the intersect two objects tool from the second box along the top
Intersect the circle and the ray
Right click on the circle, the ray and point A’ and select “show object” to remove them
Draw a segment from B to C and from A to C
Right click on the segments and choose “show value” from properties
Find the size of angle ACB and BAC
Repeat the above steps to create another triangle, this time with a segment 6 units long and
a radius of 8 units
14. Repeat the above steps again to create a third triangle, using a segment 4.5 units long and a
radius of 6 units
What do you notice about the angles of each triangle?
What do you notice about the sides of each triangle?
How can you tell if two triangles are similar?
Do not start this page until the class has discussed and answered the first page
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Draw a segment 4 units long
Draw an angle of 40° about point A
Draw a circle with centre at A and a radius of 6 units
Draw a ray from point A through B’
Intersect the circle and ray
Remove the circle, ray and point B’
Draw segments AC and BC
Label the segments AB and AC with their values
Repeat the above steps to create another triangle, this time with a segment 2 units long and
a radius of 3 units
10. Repeat the above steps again to create a third triangle, using a segment 6 units long and a
radius of 9 units
These triangles are also similar. Do they all have one angle the same?
Are the sides of each triangle in proportion?
This rule of proving two triangles are similar is like one of the rules used in congruent triangles.
Which rule is this?
Adapt the rule so that it applies for similar triangles.