The bases BC and AD of trapezoid ABCD are 4 and 64, respectively, BD = 16, Proof that triangle CBD and BDA are similar.
September 12, 2021 | education
| Let’s formally write the data from the task:
BC = 4, AD = 64, BD = 16.
Note that the angles CBD and BDA are equal, because straight lines AD and BC are parallel:
CBD = BDA = a.
Consider a triangle ABD. By the cosine theorem, we have that
AB ^ 2 = BD ^ 2 + AD ^ 2 – 2 * BD * AD * cos (a) =
= 16 ^ 2 + 64 ^ 2 – 2 * 16 * 64 * cos (a).
Similarly, from triangle BCD, we obtain
CD ^ 2 = BC ^ 2 + BD ^ 2 – 2 * BC * BD * cos (a) =
= 4 ^ 2 + 16 ^ 2 – 2 * 4 * 16 * cos (a).
Then
AB ^ 2 / CD ^ 2 = (16 ^ 2 + 64 ^ 2 – 2 * 16 * 64 * cos (a)) / (4 ^ 2 + 16 ^ 2 – 2 * 4 * 16 * cos (a)) = 4 ^ 2 and
AB / CD = 4. But BD / BC = AD / BD = 4.
So we got that for triangles CBD and BDA
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