Oblique Cylinder

Geometry Level pending

In the above Oblique Cylinder m Q P T = θ m\angle{QPT} = \theta , T P Q U TPQU is a parallelogram and P R T \triangle{PRT} is a right triangle.

Let the volume and the lateral surface area of the Oblique Cylinder be V = 1 V = 1 and A = 1 A = 1 respectively.

(1): Find the value of the radius r r which maximizes the distance y y .

(2): Find the distance d d and find θ \theta (in degrees).

Express the answer as d + θ \lfloor{d + \theta}\rfloor .


The answer is 140.

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1 solution

Rocco Dalto
Feb 9, 2018

V = π r 2 H = 1 H = 1 π r 2 V = \pi r^2 H = 1 \implies H = \dfrac{1}{\pi r^2} and A = 2 π r s = 1 s = 1 2 π r A = 2\pi rs = 1 \implies s = \dfrac{1}{2\pi r}

Y ( r ) = y 2 ( r ) = s 2 ( r ) H 2 ( r ) = 1 4 π 2 r 2 1 π 2 r 4 d Y d r = 1 π 2 ( 8 r 2 2 r 5 ) \implies Y(r) = y^2(r) = s^2(r) - H^2(r) = \dfrac{1}{4\pi^2 r^2 } - \dfrac{1}{\pi^2 r^4} \implies \dfrac{dY}{dr} = \dfrac{1}{\pi^2}(\dfrac{8 - r^2}{2r^5}) r > 0 r = 2 2 \:\ r > 0 \implies r = 2\sqrt{2} \implies s = 1 4 2 π s = \dfrac{1}{4\sqrt{2}\pi} and H = 1 8 π H = \dfrac{1}{8\pi} and y = s 2 H 2 = 1 8 π = H y = \sqrt{s^2 - H^2} = \dfrac{1}{8\pi} = H .

d 2 Y d r 2 r = 2 2 = 1 ( 8 π ) 2 < 0 \dfrac{d^2Y}{dr^2}|_{r = 2\sqrt{2}} = \dfrac{-1}{(8\pi)^2} < 0 \implies max at r = 2 2 r = 2\sqrt{2} .

Let m P T R = λ cos ( λ ) = y ( r ) s ( r ) = 1 2 m\angle{PTR} = \lambda \implies \cos(\lambda) = \dfrac{y(r)}{s(r)} = \dfrac{1}{\sqrt{2}} and θ = 180 λ cos ( θ ) = cos ( 180 θ ) = cos ( λ ) = 1 2 θ = 13 5 \theta = 180 - \lambda \implies \cos(\theta) = \cos(180 - \theta) = -\cos(\lambda) = -\dfrac{1}{\sqrt{2}} \implies \boxed{\theta = 135^\circ}

d 2 = s 2 + 4 r 2 + 4 r s cos ( λ ) = 1 32 π 2 + 32 + 2 π = \implies d^2 = s^2 + 4r^2 + 4rs \cos(\lambda) = \dfrac{1}{32\pi^2} + 32 + \dfrac{\sqrt{2}}{\pi} = 3 2 2 π 2 + 32 2 π + 1 32 π 2 \dfrac{32^2\pi^2 + 32\sqrt{2}\pi + 1}{32\pi^2} \implies d = 3 2 2 π 2 + 32 2 π + 1 4 2 π 5.69678 d = \dfrac{\sqrt{32^2\pi^2 + 32\sqrt{2}\pi + 1}}{4\sqrt{2}\pi} \approx \boxed{5.69678}

d + θ = 140 \therefore \lfloor{d + \theta}\rfloor = \boxed{140}

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