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Geometry Difficulty 4.4 AIME Find the answer

Faces ABCABC^{}_{} and BCDBCD^{}_{} of tetrahedron ABCDABCD^{}_{} meet at an angle of 3030^\circ. The area of face ABCABC^{}_{} is 120120^{}_{}, the area of face BCDBCD^{}_{} is 8080^{}_{}, and BC=10BC=10^{}_{}. Find the volume of the tetrahedron.

A number or a short expression. Spacing and $ signs are ignored.

Solutions — 2

Solution 1

Since the area BCD=80=121016BCD=80=\frac{1}{2}\cdot10\cdot16, the perpendicular from DD to BCBC has length 1616.
The perpendicular from DD to ABCABC is 16sin30=816 \cdot \sin 30^\circ=8. Therefore, the volume is 81203=320\frac{8\cdot120}{3}=\boxed{320}.

Solution 2

1. Identify the given information:
- The angle between faces ABCABC and BCDBCD is 3030^\circ.
- The area of face ABCABC is 120120.
- The area of face BCDBCD is 8080.
- The length of edge BCBC is 1010.

2. **Calculate the height of triangle BCDBCD relative to side BCBC:**
- The area of triangle BCDBCD is given by:
AreaBCD=12×BC×heightBCD \text{Area}_{BCD} = \frac{1}{2} \times BC \times \text{height}_{BCD}
- Plugging in the known values:
80=12×10×heightBCD 80 = \frac{1}{2} \times 10 \times \text{height}_{BCD}
- Solving for heightBCD\text{height}_{BCD}:
heightBCD=80×210=16 \text{height}_{BCD} = \frac{80 \times 2}{10} = 16

3. **Determine the altitude of the tetrahedron to face ABCABC:**
- The height of the tetrahedron from vertex DD perpendicular to face ABCABC can be found using the sine of the angle between the faces ABCABC and BCDBCD.
- Given that sin30=12\sin 30^\circ = \frac{1}{2}:
hABC=16sin30=16×12=8 h_{ABC} = 16 \sin 30^\circ = 16 \times \frac{1}{2} = 8

4. Calculate the volume of the tetrahedron:
- The volume VV of a tetrahedron is given by:
V=13×base area×height V = \frac{1}{3} \times \text{base area} \times \text{height}
- Here, the base area is the area of face ABCABC, which is 120120, and the height is hABCh_{ABC}, which is 88:
V=13×120×8=13×960=320 V = \frac{1}{3} \times 120 \times 8 = \frac{1}{3} \times 960 = 320

The final answer is 320\boxed{320}

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.