A quadrilateral \(ABCD\) has \(AB=8\,\text{cm}\), \(BC=4\,\text{cm}\), \(CD=5\,\text{cm}\), \(\angle A=30^\circ\), and diagonal \(BD=5\,\text{cm}\).
1. How many noncongruent triangles \(ABD\) satisfy the measurements? Explain.
2. Up to congruence, how many quadrilaterals satisfy all five measurements if self-intersecting quadrilaterals are allowed? Explain the count.
Hints
- Treat \(\triangle ABD\) as an SSA ambiguous case.
- Use the Law of Sines and test both inverse-sine angles.
- Once the shape of \(\triangle ABD\) is fixed, how many sides of \(BD\) can contain the SSS triangle \(BCD\)?
Solution
1. In \(\triangle ABD\), \(BD=5\,\text{cm}\) is opposite \(\angle A=30^\circ\), and \(AB=8\,\text{cm}\).
2. By the Law of Sines, \(\sin\angle D=\frac{8\sin30^\circ}{5}=0.8\). Therefore, \(\angle D\) can be approximately \(53.1^\circ\) or \(126.9^\circ\). Both fit with \(\angle A=30^\circ\), so there are two noncongruent possibilities for \(\triangle ABD\).
3. For either triangle \(ABD\), triangle \(BCD\) has side lengths \(4\,\text{cm}\), \(5\,\text{cm}\), and \(5\,\text{cm}\), so it is determined by SSS up to reflection across \(BD\).
4. For each of the two noncongruent choices for \(\triangle ABD\), the SSS triangle \(BCD\) can be attached on either side of \(BD\). Thus, there are \(2\cdot2=4\) quadrilaterals up to congruence.
Answer
1. There are two noncongruent possibilities for \(\triangle ABD\) because the SSA data produce two valid triangles.
2. There are \(4\) quadrilaterals up to congruence: two choices for \(\triangle ABD\), and for each one, two choices for which side of \(BD\) contains \(C\).