 # 172 geometry/corner.p

## Description

This article is from the Puzzles FAQ, by Chris Cole chris@questrel.questrel.com and Matthew Daly mwdaly@pobox.com with numerous contributions by others.

# 172 geometry/corner.p

A hallway of width A turns through 90 degrees into a hallway of width
B. A ladder is to be passed around the corner. If the movement is
within the horizontal plane, what is the maximum length of the ladder?

geometry/corner.s

```------\---------
B   /  \.......|..B/sin(theta)
theta\      |
---|-----X     |
|\    |
| \...|..A/cos(theta)
|  \  |
|   \ |
| A  \|
```

Theta is the angle off horizontal.

Minimize length = A/cos(theta) + B/sin(theta)

```    d(length)/d(theta)
= A*sin(theta)/cos(theta)^2 - B*cos(theta)/sin(theta)^2 (?)
= 0
A*sin(theta)/cos(theta)^2 = B*cos(theta)/sin(theta)^2

B/A = sin(theta)^3/cos(theta()^3 = tan(theta)^3

theta = inverse_tan(cube_root(B/A))
```

If you use the trigonometric formulas cos^2 x = 1/(1 + tan^2 x)
and sin x = tan x cos x, and plug through the algebra, I believe
that the formula for the length reduces to

(A^(2/3) + B^(2/3))^(3/2)

At any rate this is symmetric in A and B as one would expect, and
has the right values at A=B and as either A-->0 or B-->0.

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