Rectangular Pyramid

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Inside rectangular base A B C D ABCD , B E C \triangle{BEC} and A F D \triangle{AFD} are right triangles and F F lies on E C EC , where B E = 7 , E C = 24 BE = 7, EC = 24 and A F = 15 AF = 15 .

Let O O be the center of rectangle A B C D ABCD and construct a circle centered at O O whose circumference is equal to the perimeter of the rectangle. Folding the arc of the semi-circle at a right angle, as shown above, the radius of the semicircle becomes the height of the rectangular pyramid.

Find the total surface area of the rectangular pyramid.


The answer is 1112.00931858.

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2 solutions

Let F D C = F A D = α , F C D = C B E = β \angle {FDC}=\angle {FAD}=α,\angle {FCD}=\angle {CBE}=β . Then B C = A D = 7 2 + 2 4 2 = 25 , F D = 2 5 2 1 5 2 = 20 |\overline {BC}|=|\overline {AD}|=\sqrt {7^2+24^2}=25,|\overline {FD}|=\sqrt {25^2-15^2}=20 , sin α = 20 25 = 4 5 cos α = 3 5 \sin α=\dfrac{20}{25}=\dfrac{4}{5}\implies \cos α=\dfrac{3}{5} , sin β = 24 25 cot β = 7 24 \sin β=\dfrac{24}{25}\implies \cot β=\dfrac{7}{24} . C D = F D × sin ( α + β ) sin β = 20 × ( cos α + cot β sin α ) = 50 3 |\overline {CD}|=|\overline {FD}|\times \dfrac{\sin (α+β)}{\sin β}=20\times (\cos α+\cot β\sin α)=\dfrac{50}{3} . From the given condition, the height of the pyramid h h is given by 2 π h = 2 ( 50 3 + 25 ) h = 125 3 π 2πh=2\left (\dfrac{50}{3}+25\right )\implies h=\dfrac{125}{3π} . Height of the triangle on base A D \overline {AD} is ( 125 3 π ) 2 + ( 25 3 ) 2 15.6636 \sqrt {\left (\dfrac{125}{3π}\right ) ^2+\left (\dfrac{25}{3}\right) ^2}\approx 15.6636 , and on base C D \overline {CD} is ( 125 3 π ) 2 + ( 25 2 ) 2 18.2251 \sqrt {\left (\dfrac{125}{3π}\right) ^2+\left (\dfrac{25}{2}\right) ^2}\approx 18.2251 . Area of base = 25 × 50 3 416.667 25\times \dfrac{50}{3}\approx 416.667 . Lateral surface area is 2 × 1 2 × 15.6636 + 2 × 1 2 × 18.2251 = 695.3426 2\times \dfrac{1}{2}\times 15.6636+2\times \dfrac{1}{2}\times 18.2251=695.3426 . Hence the total surface area is 416.667 + 695.3426 = 1112.0093 416.667+695.3426=\boxed {1112.0093} .

Rocco Dalto
Apr 1, 2020

Let B E = 7 , E C = 24 BE = 7, EC = 24 and A F = 15 AF = 15 .

B C = A D = 7 2 + 2 4 2 = 625 = 25 BC = AD = \sqrt{7^2 + 24^2} = \sqrt{625} = 25 and 225 = 25 A G A G = 9 225 = 25 * AG \implies AG = 9

G D = H C = 16 h 1 = 1 5 2 9 2 = 144 = 12 \implies GD = HC = 16 \implies h_{1} = \sqrt{15^2 - 9^2} = \sqrt{144} = 12

B E C H F C 7 h 2 24 16 h 2 = 14 3 \triangle{BEC} \sim \triangle{HFC} \implies \dfrac{7}{h_{2}} - \dfrac{24}{16} \implies h_{2} = \dfrac{14}{3} \implies

A B = C D = 12 + 14 3 = 50 3 A A B C D = 25 ( 50 3 ) = 1250 3 AB = CD = 12 + \dfrac{14}{3} = \dfrac{50}{3} \implies A_{ABCD} = 25(\dfrac{50}{3}) = \dfrac{1250}{3}

The perimeter P = 50 + 100 3 = 250 3 = 2 π r r = 125 3 π = O P P = 50 + \dfrac{100}{3} = \dfrac{250}{3} = 2\pi r \implies r = \dfrac{125}{3\pi} = OP

O T = 25 3 OT = \dfrac{25}{3} and O S = 25 2 OS = \dfrac{25}{2} and O P = 125 3 π OP = \dfrac{125}{3\pi} \implies

P T = 25 3 π π 2 + 25 PT = \dfrac{25}{3\pi}\sqrt{\pi^2 + 25} and P S = 25 6 π 9 π 2 + 100 PS = \dfrac{25}{6\pi}\sqrt{9\pi^2 + 100} \implies

The total surface area A = 1250 3 + 625 3 π π 2 + 25 + 625 9 π π 2 + 100 = A = \dfrac{1250}{3} + \dfrac{625}{3\pi}\sqrt{\pi^2 + 25} + \dfrac{625}{9\pi}\sqrt{\pi^2 + 100} =

625 9 π ( 6 π + 3 π 2 + 25 + 9 π 2 + 100 ) 1112.00931858 \dfrac{625}{9\pi}(6\pi + 3\sqrt{\pi^2 + 25} + \sqrt{9\pi^2 + 100}) \approx \boxed{1112.00931858} .

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