Inside rectangular base A B C D , △ B E C and △ A F D are right triangles and F lies on E C , where B E = 7 , E C = 2 4 and A F = 1 5 .
Let O be the center of rectangle A B C D and construct a circle centered at 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.
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Let B E = 7 , E C = 2 4 and A F = 1 5 .
B C = A D = 7 2 + 2 4 2 = 6 2 5 = 2 5 and 2 2 5 = 2 5 ∗ A G ⟹ A G = 9
⟹ G D = H C = 1 6 ⟹ h 1 = 1 5 2 − 9 2 = 1 4 4 = 1 2
△ B E C ∼ △ H F C ⟹ h 2 7 − 1 6 2 4 ⟹ h 2 = 3 1 4 ⟹
A B = C D = 1 2 + 3 1 4 = 3 5 0 ⟹ A A B C D = 2 5 ( 3 5 0 ) = 3 1 2 5 0
The perimeter P = 5 0 + 3 1 0 0 = 3 2 5 0 = 2 π r ⟹ r = 3 π 1 2 5 = O P
O T = 3 2 5 and O S = 2 2 5 and O P = 3 π 1 2 5 ⟹
P T = 3 π 2 5 π 2 + 2 5 and P S = 6 π 2 5 9 π 2 + 1 0 0 ⟹
The total surface area A = 3 1 2 5 0 + 3 π 6 2 5 π 2 + 2 5 + 9 π 6 2 5 π 2 + 1 0 0 =
9 π 6 2 5 ( 6 π + 3 π 2 + 2 5 + 9 π 2 + 1 0 0 ) ≈ 1 1 1 2 . 0 0 9 3 1 8 5 8 .
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Let ∠ F D C = ∠ F A D = α , ∠ F C D = ∠ C B E = β . Then ∣ B C ∣ = ∣ A D ∣ = 7 2 + 2 4 2 = 2 5 , ∣ F D ∣ = 2 5 2 − 1 5 2 = 2 0 , sin α = 2 5 2 0 = 5 4 ⟹ cos α = 5 3 , sin β = 2 5 2 4 ⟹ cot β = 2 4 7 . ∣ C D ∣ = ∣ F D ∣ × sin β sin ( α + β ) = 2 0 × ( cos α + cot β sin α ) = 3 5 0 . From the given condition, the height of the pyramid h is given by 2 π h = 2 ( 3 5 0 + 2 5 ) ⟹ h = 3 π 1 2 5 . Height of the triangle on base A D is ( 3 π 1 2 5 ) 2 + ( 3 2 5 ) 2 ≈ 1 5 . 6 6 3 6 , and on base C D is ( 3 π 1 2 5 ) 2 + ( 2 2 5 ) 2 ≈ 1 8 . 2 2 5 1 . Area of base = 2 5 × 3 5 0 ≈ 4 1 6 . 6 6 7 . Lateral surface area is 2 × 2 1 × 1 5 . 6 6 3 6 + 2 × 2 1 × 1 8 . 2 2 5 1 = 6 9 5 . 3 4 2 6 . Hence the total surface area is 4 1 6 . 6 6 7 + 6 9 5 . 3 4 2 6 = 1 1 1 2 . 0 0 9 3 .