Oblique Cylinder 3

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 = b 2 V = b^2 and A = b A = b respectively, where b b is a positive real number.

(1): Find the restriction on b b for which the radius r r minimizes the distance y y .

(2): Find θ \theta in degrees.

If r < α r < \alpha and d < β d < \beta , find α \lfloor{\alpha}\rfloor + β + θ \lfloor{\beta}\rfloor + \theta .


The answer is 139.

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1 solution

Rocco Dalto
Feb 11, 2018

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

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

For r r to minimize the distance y y d 2 Y d r 2 r = 2 2 b > 0 d 2 Y d r 2 r = 2 2 b = 1 128 π 2 ( 3 5 b ) > 0 b < 3 5 r < 6 2 5 = α 1.697056 \dfrac{d^2Y}{dr^2}|_{r = 2\sqrt{2} b} > 0 \implies \dfrac{d^2Y}{dr^2}|_{r = 2\sqrt{2} b} = \dfrac{1}{128\pi^2}(3 - 5b) > 0 \implies \boxed{b < \dfrac{3}{5}} \implies \boxed{r < \dfrac{6\sqrt{2}}{5} = \alpha \approx 1.697056} .

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 b 2 + 2 b π < 1 32 π 2 + 32 ( 9 25 ) + 2 π ( 3 5 ) = \implies d^2 = s^2 + 4r^2 + 4rs \cos(\lambda) = \dfrac{1}{32\pi^2} + 32 b^2 + \dfrac{\sqrt{2} b}{\pi} < \dfrac{1}{32\pi^2} + 32 (\dfrac{9}{25}) + \dfrac{\sqrt{2}}{\pi}(\dfrac{3}{5}) =

( 96 π ) 2 + 480 2 π + 25 2 ( 20 π ) 2 d < ( 96 π ) 2 + 480 2 π + 25 20 2 π = β 3.43413 \dfrac{(96\pi)^2 + 480\sqrt{2}\pi + 25}{2(20\pi)^2} \implies \boxed{d < \dfrac{\sqrt{(96\pi)^2 + 480\sqrt{2}\pi + 25}}{20\sqrt{2}\pi} = \beta \approx 3.43413}

α \therefore \lfloor{\alpha}\rfloor + β + θ = 139 \lfloor{\beta}\rfloor + \theta = \boxed{139}

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