1999 AIME Problem 15

Below is the professionally curated solution for Problem 15 of the 1999 AIME, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1999 AIME solutions, or check the answer key.

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Concepts:paper folding3D geometrycoordinate geometryvolume

Difficulty rating: 2990

15.

Consider the paper triangle whose vertices are (0,0),(0, 0), (34,0),(34, 0), and (16,24).(16, 24). The vertices of its midpoint triangle are the midpoints of its sides. A triangular pyramid is formed by folding the triangle along the sides of its midpoint triangle. What is the volume of this pyramid?

Solution:

The midpoints are M1=(17,0),M_1 = (17, 0), M2=(25,12),M_2 = (25, 12), and M3=(8,12).M_3 = (8, 12). Folding the three corner triangles up along the sides of the midpoint triangle brings the corners together at one apex QQ (each pair of glued half-sides has equal length). The apex keeps its folded distances: QM1=17QM_1 = 17 (half of the side of length 3434 that M1M_1 bisects), QM2=15QM_2 = 15 (half of 3030), and QM3=413QM_3 = 4\sqrt{13} (half of 162+242=813\sqrt{16^2 + 24^2} = 8\sqrt{13}).

Keep the midpoint triangle in the plane z=0z = 0 and let Q=(x,y,z).Q = (x, y, z). Subtracting QM32=208|Q - M_3|^2 = 208 from QM22=225|Q - M_2|^2 = 225 gives (x25)2(x8)2=17,(x - 25)^2 - (x - 8)^2 = 17, so x=16;x = 16; subtracting QM22=225|Q - M_2|^2 = 225 from QM12=289|Q - M_1|^2 = 289 gives 2x+3y=68,2x + 3y = 68, so y=12.y = 12. Then z2=289(1617)2122=144,z^2 = 289 - (16 - 17)^2 - 12^2 = 144, so the apex is at height z=12.z = 12.

The base is the midpoint triangle, with area one quarter of the original triangle's 123424=408,\frac{1}{2} \cdot 34 \cdot 24 = 408, i.e. 102.102. The volume is 1310212=408.\frac{1}{3} \cdot 102 \cdot 12 = 408.

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