Video Transcript
Using the diagram, which of the
following is equal to 𝐴𝐵 over 𝐴𝐷. Is it (A) 𝐴𝐶 over 𝐸𝐶 or (B)
𝐴𝐵 over 𝐷𝐵 or (C) 𝐴𝐷 over 𝐷𝐵, answer (D), 𝐴𝐶 over 𝐴𝐸, or (E) 𝐴𝐸
over 𝐸𝐶.
We see from the diagram that
the base of triangle 𝐴𝐸𝐷, that′s side 𝐸𝐷, is parallel to the base of
triangle 𝐴𝐵𝐶, that′s side 𝐶𝐵. Since corresponding angles must
be equal if the two lines are parallel, angle 𝐷𝐸𝐴 is equal to angle 𝐵𝐶𝐴,
and angle 𝐸𝐷𝐴 is equal to angle 𝐶𝐵𝐴. Then side 𝐸𝐷 creates triangle
𝐴𝐷𝐸, which is similar to the larger triangle 𝐴𝐵𝐶. Since these triangles are
similar, the ratios of their corresponding side lengths must be equal. In particular, 𝐴𝐸 over 𝐴𝐶
is equal to 𝐴𝐷 over 𝐴𝐵.
Now we want to find which of
the given fractions is equal to 𝐴𝐵 over 𝐴𝐷. And we can do this by finding
the reciprocal of both sides of our equation, which gives us 𝐴𝐶 over 𝐴𝐸 is
equal to 𝐴𝐵 over 𝐴𝐷. Hence, 𝐴𝐵 over 𝐴𝐷 is equal
to 𝐴𝐶 over 𝐴𝐸, which corresponds to the given option (D).
