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A--------------C............................B
W-x x
Ingenious as these musings are, I have decided that allowing A to wander beyond the bounds of the paper wasn’t in the spirit of the diagram. My solution (not hidden) follows then we can move on to something else. This involves trigonometry and calculus, skills which some may have forgotten.
Let BC=x, CD=c, angle BCD=theta. AC = W-x. Note 0.5*W < x <= W. Then by the nature of paper folding triangles DCB and DCB' are congruent and angle B’CD = theta making angle ACB'=pi-2*theta.
In triangle BCD cos(theta)=x/c and in triangle AB’C cos(pi-2*theta)=(W-x)/x.
So -cos(2*theta)=W/x-1, cos(2*theta)=1-W/x, 2*cos(theta)^2-1 = 1-W/x
2*cos(theta)^2 = 2-W/x = 2*(x/c)^2
Hence 2*x*c^2-W*c^2 = 2*x^3, c^2 = 2*x^3 / (2*x-W) (1)
c will attain its minimum when c^2 does so we don’t need to extract the square root before differentiating wrt x: (d/dx)(c^2) = ((2*x-W)*6*x^2 – 4*x^3) / (2*x-W)^2
The minimum is attained when the numerator is zero:
8*x^3 – 6*W*x^2 = 0 or x^2 * (8*x 6*W) = 0
x can’t be zero so the only root is x = 3*W/4. Substituting this value into (1) gives
c^2 = 2*(3/4)^3 * W^3 / (0.5 * W)
giving the minimum c = (3*sqrt(3)/4) * W
Cinelli