There's a ball in a cone. Let the radius of the ball is r cm and the radius of cone is R cm.
The height of the cone is h cm and the lateral height (the length of a line segment from the apex of the cone along its side to its base) is l cm.
If
R + h = 1 1 2
h + l = 1 4 4
l + R = 1 2 8
What is the ratio between the volume of ball and the volume of cone?
The answer is the form of x : y . Submit your answer as ( x 2 + 3 x y + y 2 ) ( x 2 + x y + y 2 ) .
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@Tapas Mazumdar Nice solution. I thought your solution is much better than mine...
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Thanks. But you solution has a nice and a different approach, and it is nice too. :)
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Thanks.. Stay tuned.. Hehehehe
I am thinking ahout the level.. How about you??
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@Fidel Simanjuntak – I would say a Level 3 or 4.
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@Tapas Mazumdar – Really? Okay, I'll confirm it.. Thanks for your suggestion
R + h = 1 1 2 . . . . . . . . . . ( 1 )
h + l = 1 4 4 . . . . . . . . . . ( 2 )
l + R = 1 2 8 . . . . . . . . . . ( 3 )
( 1 ) + ( 2 ) + ( 3 )
2 ( R + h + l ) = 3 8 4
R + h + l = 1 9 2 . . . . . . . . . . ( 4 )
( 4 ) − ( 2 )
R = 4 8 ; By Triple Pythagoras, h = 6 4 ; l = 8 0
r = 2 1 × ( 2 R + 2 l ) 2 R h × 2 1
r = 2 ( R + l ) 2 R h
r = R + l R h
r = 1 2 8 3 0 7 2
r = 2 4
Volume of Ball : Volume of Cone
3 4 × π × r ³ : 3 1 × π × R ² × h
4 × 2 4 × 2 4 × 2 4 : 4 8 × 4 8 × 6 4
3 : 8 = x : y
x = 3 ; y = 8
Note that x ² + 3 x y + y ² = x ² + 2 x y + y ² + x y = ( x + y ) ² + x y and
x ² + x y + y ² = x ² + 2 x y + y ² − x y = ( x + y ) ² − x y
Now,
( ( x + y ) ² + x y ) ( ( x + y ) ² − x y ) = ( x + y ) 4 − ( ( x y ) 2 )
1 1 4 − 2 4 2 = 1 4 6 4 1 − 5 7 6
= 1 4 0 6 5
@Tapas Mazumdar - Can you give a solution?
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R + h h + l l + R = = = 1 1 2 ⋯ ( 1 ) 1 4 4 ⋯ ( 2 ) 1 2 8 ⋯ ( 3 )
( 1 ) + ( 2 ) + ( 3 ) gives,
2 ( l + R + h ) = 3 8 4 ⟹ ( l + R + h ) = 1 9 2 ⋯ ( 4 )
( 4 ) − ( 1 ) gives,
l = 8 0
( 4 ) − ( 2 ) gives,
R = 4 8
( 4 ) − ( 3 ) gives,
h = 6 4
Consider the diagram above of half-conical section where θ is exactly half of the original angle made at the apex.
Using trigonometry, we have,
cos θ = l h = 8 0 6 4 = 5 4 ∴ sin θ = 1 − cos 2 θ = 5 3
From the figure, we observe that,
sin θ = h − r r ∴ h − r r = 5 3 ⟹ 6 4 − r r = 5 3 ⟹ r = 2 4
Thus,
Volume of cone Volume of ball = = = = = 3 1 π R 2 h 3 4 π r 3 4 8 2 ⋅ 6 4 1 6 4 1 ⋅ 2 4 3 2 4 2 1 ⋅ 2 2 ⋅ 1 6 2 4 3 2 4 6 4 2 4 8 3 = y x
Hence,
( x 2 + 3 x y + y 2 ) ( x 2 + x y + y 2 ) = [ ( x + y ) 2 + x y ] [ ( x + y ) 2 − x y ] = ( x + y ) 4 − ( x y ) 2 = ( 3 + 8 ) 4 − ( 3 × 8 ) 2 = 1 1 4 − 2 4 2 = 1 4 6 4 1 − 5 7 6 = 1 4 0 6 5