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Product of the intercepts of intersecting secants

In the diagram, the secants CE and AE intersect at the point E. Change the orientation and shape of the diagram by clicking and dragging any of the blue points. What do you observe?

Consider the two triangles CEB and AED.

1. Why is <ECB = <EAD?
2. Given that <CEB = <AED (common angle), and using (1) above, we can say that the triangles CBE and ADE are similar. Why is this so?
3. Given that the corresponding sides in similar triangles are in the same ratio, we can say that CE/AE = BE/DE. How can we use this result to prove that the product of the intersecting secants are equal?

© State of New South Wales, Department of Education, 2018