How high ?!

Geometry Level pending

Polyhedron A B C D ABCD - E F G H EFGH has the bottom rectangle A B C D ABCD measuring 56 × 36 56 \times 36 and top rectangle E F G H EFGH measuring 32 × 24 32 \times 24 . The two rectangles are aligned, with the longer edges parallel to the x x -axis, and the shorter edges parallel to y y -axis, and their centers lying on the vertical z z -axis with a vertical separation of h = 24 h = 24 .

Now the planes connecting the two rectangles are extended, so that they form the polyhedron E F G H EFGH - I J IJ on top of rectangle E F G H EFGH . How high above the plane of rectangle A B C D ABCD are points I I and J J ? (they are at the same elevation)


The answer is 56.

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1 solution

Hosam Hajjir
Nov 18, 2020

Taking a horizontal cross section of polyhedron E F G H I J EFGH-IJ at elevation z z , it will be a rectangle having dimensions L ( z ) × W ( z ) L(z) \times W(z)

The length L ( z ) = 56 + ( 32 56 ) h z = 56 z L(z) = 56 + \dfrac{(32 -56)}{h} z = 56 - z , and the width W ( z ) = 36 + 24 36 h z = 36 1 2 z W(z) = 36 + \dfrac{24 - 36 }{h} z = 36 - \frac{1}{2} z

Since L ( z ) L(z) vanishes before W ( z ) W(z) (for a less value of z z ), it follows that point I I and J J are at an elevation of z = 56 z =\boxed{ 56} .

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