A Ball in a Cone? Is that something new?

Geometry Level 3

There's a ball in a cone. Let the radius of the ball is r r cm and the radius of cone is R R cm.

The height of the cone is h 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 l cm.

If

R + h = 112 R+h= 112

h + l = 144 h+l= 144

l + R = 128 l+R = 128

What is the ratio between the volume of ball and the volume of cone?

The answer is the form of x : y x:y . Submit your answer as ( x 2 + 3 x y + y 2 ) ( x 2 + x y + y 2 ) (x^2+3xy+y^2)(x^2+xy+y^2) .

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The answer is 14065.

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2 solutions

Tapas Mazumdar
Sep 28, 2016

R + h = 112 ( 1 ) h + l = 144 ( 2 ) l + R = 128 ( 3 ) \begin{aligned} R+h & = & 112 \quad \cdots (1) \\ h+l & = & 144 \quad \cdots (2) \\ l+R & = & 128 \quad \cdots (3) \end{aligned}

( 1 ) + ( 2 ) + ( 3 ) (1)+(2)+(3) gives,

2 ( l + R + h ) = 384 ( l + R + h ) = 192 ( 4 ) 2 \left( l+R+h \right) = 384 \implies \left( l+R+h \right) = 192 \quad \cdots (4)

( 4 ) ( 1 ) (4) - (1) gives,

l = 80 \color{#D61F06}{l=80}

( 4 ) ( 2 ) (4) - (2) gives,

R = 48 \color{#20A900}{R=48}

( 4 ) ( 3 ) (4) - (3) gives,

h = 64 \color{#302B94}{h=64}

Consider the diagram above of half-conical section where θ \theta is exactly half of the original angle made at the apex.

Using trigonometry, we have,

cos θ = h l = 64 80 = 4 5 sin θ = 1 cos 2 θ = 3 5 \cos \theta = \dfrac{\color{#302B94}{h}}{\color{#D61F06}{l}} = \dfrac{64}{80} = \dfrac{4}{5} \\ \therefore \sin \theta = \sqrt{1- \cos^2 \theta} = \dfrac{3}{5}

From the figure, we observe that,

sin θ = r h r r h r = 3 5 r 64 r = 3 5 r = 24 \sin \theta = \dfrac{r}{\color{#302B94}{h}-r} \\ \therefore \dfrac{r}{\color{#302B94}{h}-r} = \dfrac 35 \implies \dfrac{r}{64-r} = \dfrac 35 \implies \color{maroon}{r = 24}

Thus,

Volume of ball Volume of cone = 4 3 π r 3 1 3 π R 2 h = 4 1 2 4 3 4 8 2 64 16 = 2 4 3 24 2 4 2 1 2 2 16 = 24 64 = 3 8 = x y \begin{aligned} \dfrac{\text{Volume of ball}}{\text{Volume of cone}} &=& \dfrac{\frac {4}{\cancel{3}} \cancel{\pi} r^3}{\frac{1}{\cancel{3}} \cancel{\pi} R^2 h} \\ \\ &=& \dfrac{\cancel{4}^1 \cdot 24^3}{48^2 \cdot \cancel{64}^{16}} \\ \\ &=& \dfrac{\cancel{24^3}^{24}}{\cancel{24^2}^1 \cdot 2^2 \cdot 16} \\ \\ &=& \dfrac{24}{64} \\ \\ &=& \dfrac 38 = \dfrac{\color{#D61F06}{x}}{\color{#3D99F6}{y}} \end{aligned}

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 = 14641 576 = 14065 \begin{aligned} \left( x^2+3xy+y^2 \right) \left(x^2+xy+y^2 \right) &= \left[ {\left(x+y\right)}^2 + xy \right] \left[ {\left(x+y\right)}^2 - xy \right] \\ &= {\left(\color{#D61F06}{x}+\color{#3D99F6}{y}\right)}^4 - {(\color{#D61F06}{x} \color{#3D99F6}{y})}^2 \\ & = {(3+8)}^4 - {(3 \times 8)}^2 \\ & = 11^4 - 24^2 \\ & = 14641-576 \\ &= \boxed{14065} \end{aligned}

@Tapas Mazumdar Nice solution. I thought your solution is much better than mine...

Fidel Simanjuntak - 4 years, 8 months ago

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Thanks. But you solution has a nice and a different approach, and it is nice too. :)

Tapas Mazumdar - 4 years, 8 months ago

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Thanks.. Stay tuned.. Hehehehe

Fidel Simanjuntak - 4 years, 8 months ago

I am thinking ahout the level.. How about you??

Fidel Simanjuntak - 4 years, 8 months ago

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@Fidel Simanjuntak I would say a Level 3 or 4.

Tapas Mazumdar - 4 years, 8 months ago

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@Tapas Mazumdar Really? Okay, I'll confirm it.. Thanks for your suggestion

Fidel Simanjuntak - 4 years, 8 months ago
Fidel Simanjuntak
Sep 28, 2016

R + h = 112 R+h =112 . . . . . . . . . . ( 1 ) ..........(1)

h + l = 144 h+l=144 . . . . . . . . . . ( 2 ) ..........(2)

l + R = 128 l+R=128 . . . . . . . . . . ( 3 ) ..........(3)

( 1 ) + ( 2 ) + ( 3 ) (1) + (2) + (3)

2 ( R + h + l ) = 384 2(R+h+l) = 384

R + h + l = 192 R+h+l=192 . . . . . . . . . . ( 4 ) ..........(4)

( 4 ) ( 2 ) (4) - (2)

R = 48 R=48 ; By Triple Pythagoras, h = 64 ; l = 80 h = 64 ; l=80

r = 2 R h × 1 2 1 2 × ( 2 R + 2 l ) r = \frac{2Rh\times \frac{1}{2}}{\frac{1}{2}\times(2R+2l)}

r = 2 R h 2 ( R + l ) r= \frac{2Rh}{2(R+l)}

r = R h R + l r= \frac{Rh}{R+l}

r = 3072 128 r = \frac{3072}{128}

r = 24 r=24

Volume of Ball : : Volume of Cone

4 3 × π × r ³ : 1 3 × π × R ² × h \frac{4}{3} \times \pi \times r³ : \frac{1}{3} \times \pi \times R² \times h

4 × 24 × 24 × 24 : 48 × 48 × 64 4 \times 24 \times 24 \times 24 : 48 \times 48 \times 64

3 : 8 = x : y 3:8 = x:y

x = 3 ; y = 8 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 x² + 3xy + y² = x² + 2xy + y² + xy = (x+y)² + xy and

x ² + x y + y ² = x ² + 2 x y + y ² x y = ( x + y ) ² x y x² + xy + y² = x² + 2xy + y² - xy = (x+y)²-xy

Now,

( ( x + y ) ² + x y ) ( ( x + y ) ² x y ) = ( x + y ) 4 ( ( x y ) 2 ) ((x+y)² +xy )( (x+y)² -xy) = (x+y)^4 -((xy)^2)

1 1 4 2 4 2 = 14641 576 11^4 - 24^2 = 14641 -576

= 14065 = \boxed{14065}

@Tapas Mazumdar - Can you give a solution?

Fidel Simanjuntak - 4 years, 8 months ago

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