In this above rocket picture,
The blue line serves as the larger radius of the lampshade portion, and has a length of 2 inches.
If the surface area of the rocket (in square inches) equals S A , find ⌊ S A ⌋ .
Note: This "rocket" has a sealed base. Please include the base in your answer.
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CSA of the cone portion = π r l = π ⋅ 1 ⋅ h 2 + r 2 = π 2 6
CSA of cylinder portion = 2 π r h = 2 π ⋅ 1 ⋅ 1 0 = 2 0 π
CSA of frustum portion = π ( r 1 + r 2 ) h 2 + ( r 1 − r 2 ) 2 + π r 2 2 = 3 π 2 6 + 4 π
The Total SA, = 4 π 2 6 + 2 4 π ∼ 1 3 9 . 4 7 ⇒ ⌊ 1 3 9 . 4 7 ⌋ = 1 3 9
The point of this problem was that you didn't need the frustrum portion. You just needed to combine the top cone with the bottom frustrum to get a bigger cone. The ratios add up.
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Aah !! I can see it now !
We don't even need the radius of the cone thingy on the top, we only need similarities to prove that it is half of the radius of the lampshade thingy at the bottom.
Dont you think that was a good practice for Boards Summatives..!! Ahhh....
But did you include the base? It is a rocket and has an open base.
The two conical bits (frustum at the bottom and cone at the top) have the same height, and the radius of the top cone is half the radius of the bottom of the frustum. Proportions therefore allow us to put these two pieces together to produce a cone of height 2*5 = 10. The slant length, l , of this cone is 1 0 2 + 2 2 , by Pythagoras' Theorem. Since the curved surface area of a cone is π r l , we find that it is 2 π 1 0 2 + 2 2 .
Next, the area of a circle is π r 2 , so the area of the base is 4 π .
Finally the curved surface area of a cylinder is π r 2 h . This is 2 0 π here.
The total is 2 π 1 0 4 + 4 π + 2 0 π .
This value is just short of 1 4 0 , hence the answer of 1 3 9 .
you should ve wrote that you must round it
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Relevant wiki: Surface Area - Problem Solving
NOTE: The figure above is not drawn to scale.
Let A be the surface area of the top part, B be the surface area of the middle part and C be the surface area of the bottom part. Then,
A = 2 1 c L = 2 1 ( π ) ( 2 ) 5 2 + 1 2 = π 2 6
B = c h = 2 π ( 1 0 ) = 2 0 π
C = 2 1 ( c 1 + c 2 ) L + π r 2 = 2 1 π ( 2 + 4 ) 5 2 + 1 2 + π ( 2 2 ) = 2 1 π ( 6 ) 2 6 + 4 π = 3 π 2 6 + 4 π
Finally,
S A = A + B + C = π 2 6 + 2 0 π + 3 π 2 6 + 4 π ≈ 1 3 9 a n s w e r
Definitions:
c = circumference of the circle
L = slant height
r = radius of the circle
h = height
c 1 = upper circumference
c 2 = lower circumference