Comparing.

Level pending

Let n 4 n \ge 4 be a positive integer and P n P_{n} be a pyramid whose base is a regular n n -gon.

Let θ \theta be the angle of inclination (in degrees) made between the slant height and the base which minimizes the lateral surface area of the pyramid P n P_{n} when the volume is held constant.

Find the angle θ \theta and show the angle θ \theta is independent of n n .

Let x ( t ) = a t 2 + b , y ( t ) = c t 3 + d x(t) = at^2 + b, \:\ y(t) = ct^3 +d .

There are two lines which are both tangent and normal to the above curve.

Find the angle λ \lambda (in degrees) made between the two lines above.

Find λ θ \dfrac{\lambda}{\theta} .

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The answer is 2.

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1 solution

Rocco Dalto
Oct 8, 2018

For area of n g o n n - gon :

Let B C = x BC = x be a side of the n g o n n - gon , A C = A B = r AC = AB= r , A D = h AD = h^* , and B A D = 180 n \angle{BAD} = \dfrac{180}{n} .

x 2 = r sin ( 180 n ) r = x 2 sin ( 180 n ) h = x 2 cot ( 180 n ) A A B C = 1 4 cot ( 180 n ) x 2 \dfrac{x}{2} = r \sin(\dfrac{180}{n}) \implies r = \dfrac{x}{2 \sin(\dfrac{180}{n})} \implies h^* = \dfrac{x}{2} \cot(\dfrac{180}{n}) \implies A_{\triangle{ABC}} = \dfrac{1}{4} \cot(\dfrac{180}{n}) x^2 \implies

A n g o n = n 4 cot ( 180 n ) x 2 A_{n - gon} = \dfrac{n}{4} \cot(\dfrac{180}{n}) x^2 \implies the Volume of the pyramid V p = n 12 cot ( 180 n ) x 2 H V_{p} = \dfrac{n}{12} \cot(\dfrac{180}{n}) x^2 H

The lateral surface area P = n x 2 s , P^{*} = \dfrac{nx}{2} * s, where s s is the slant height.

V p = K V_{p} = K (constant) H = 12 K n c o t ( 180 n ) x 2 \implies H = \dfrac{12 K}{n * cot(\dfrac{180}{n}) x^2} \implies P ( x ) = 1 2 m ( n ) x ( j ( n ) m ( n ) ) 2 x 6 + ( 24 K ) 2 P^{*}(x) = \dfrac{1}{2 m(n) x} \sqrt{(j(n) * m(n))^2 x^6 + (24 K)^2} , where m ( n ) = cot ( 180 n ) m(n) = \cot(\dfrac{180}{n}) and j ( n ) = n m ( n ) j(n) = n * m(n)

d P d x = 2 ( j ( n ) m ( n ) ) 2 x 6 ( 24 K ) 2 2 m ( n ) ( j ( n ) m ( n ) ) 2 x 6 + ( 24 K ) 2 x 2 = 0 \implies \dfrac{dP^{*}}{dx} = \dfrac{2 (j(n) * m(n))^2 x^6 - (24 K)^2}{2 m(n) \sqrt{(j(n) * m(n))^2 x^6 + (24 K)^2} * x^2} = 0 x = ( ( 24 K ) 2 2 ( j ( n ) m ( n ) ) 2 ) 1 6 \implies x = (\dfrac{(24 K)^2}{2 (j(n)* m(n))^2})^{\dfrac{1}{6}}

H = 12 K ( j ( n ) ) ( ( 24 K ) 2 2 ( j ( n ) m ( n ) ) 2 ) 1 3 \implies H = \dfrac{12 K}{(j(n)) *( \dfrac{(24 K)^2}{2 (j(n) * m(n))^2})^{\dfrac{1}{3}}}

tan ( θ ) = H h = 2 θ = arctan ( 2 ) 54.73561. \implies \tan(\theta) = \dfrac{H}{h^{*}} = \sqrt{2} \implies \theta = \arctan(\sqrt{2}) \approx 54.73561 .

Let x ( t ) = a t 2 + b , y ( t ) = c t 3 + d d y d x ( t = t 1 ) = 3 c 2 a t 1 x(t) = at^2 + b,\:\ y(t) = ct^3 + d \implies \dfrac{dy}{dx}|(t = t_{1}) = \dfrac{3c}{2a}t_{1} \implies the tangent line to the curve at ( x ( t 1 ) , y ( t 1 ) ) (x(t_{1}),y(t_{1})) is: y ( c t 1 3 + d ) = 3 c 2 a t 1 ( x ( a t 1 2 + b ) ) y - (ct_{1}^3 + d) = \dfrac{3c}{2a}t_{1}(x - (at_{1}^2+ b))

Let the line be normal to the curve at ( x ( t 2 ) , y ( t 2 ) ) c ( t 2 t 1 ) ( t 2 2 + t 1 t 2 + t 1 2 ) = 3 c 2 t 1 ( t 2 t 1 ) ( t 2 + t 1 ) c 2 ( t 2 t 1 ) ( 2 t 2 2 t 1 t 2 t 1 2 ) = 0 (x(t_{2}),y(t_{2})) \implies c(t_{2} - t_{1})(t_{2}^2 + t_{1}t_{2} + t_{1}^2) = \dfrac{3c}{2}t_{1}(t_{2} - t_{1})(t_{2} + t_{1}) \implies \dfrac{c}{2}(t_{2} - t_{1})(2t_{2}^2 - t_{1}t_{2} - t_{1}^2) = 0 t 1 t 2 t 2 = t 1 2 t_{1} \neq t_{2} \implies t_{2} = -\dfrac{t_{1}}{2}

Since the tangent is also normal to the curve at ( x ( t 2 ) , y ( t 2 ) ) 9 c 2 4 a 2 t 1 t 2 = 1 (x(t_{2}),y(t_{2})) \implies \dfrac{9c^2}{4a^2}t_{1}t_{2} = -1 9 c 2 8 a 2 t 1 2 = 1 t 1 = ± 2 2 a 3 c \implies \dfrac{9c^2}{8a^2}t_{1}^2 = 1 \implies t_{1} = \pm\dfrac{2\sqrt{2}a}{3c} \implies the two slopes are ± 2 \pm\sqrt{2} .

tan ( θ ) = 2 θ = arctan ( 2 ) 54.73561 λ = 2 θ 109.47122 \tan(\theta) = \sqrt{2} \implies \theta = \arctan(\sqrt{2}) \approx 54.73561 \implies \lambda = 2\theta \approx \boxed{109.47122} .

λ θ = 2 θ θ = 2 \implies \dfrac{\lambda}{\theta} = \dfrac{2\theta}{\theta} = \boxed{2}

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