Calculative or not?

Geometry Level 3

A right triangle, whose base is 24 and height is 18, is made to revolve along its base and height respectively. Find the ratio of volumes of the two solids so obtained.

3:4 Can't be calculated 1:1 4:3

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3 solutions

Shubham Poddar
Dec 7, 2017

If the base and height of the right triangle be b and h respectively then, The volume when the triangle is rotated about its base will be = 1/3πh^2b. And the volume when the triangle is rotated about its height will be = 1/3πb^2h. Hence the ratio of the volumes will be = h/b. In this case, the ratio of volumes will be = 18/24 = 3/4. So it is not calculative.

After revolving, it forms two regular cones. Solving for their volumes

V 1 = 1 3 π ( 18 ) 2 ( 24 ) = 1 3 π ( 7766 ) V_1 = \frac{1}{3}\pi(18)^2(24) = \frac{1}{3}\pi(7766)

V 2 = 1 3 π ( 24 ) 2 ( 18 ) = 1 3 π ( 10368 ) V_2 = \frac{1}{3}\pi(24)^2(18) = \frac{1}{3}\pi(10368)

Solving for the ratio of their volumes

V 1 V 2 = 7776 10368 = 3 4 \frac{V_1}{V_2} = \frac{7776}{10368} = \frac{3}{4}

Toshit Jain
Mar 30, 2017

G i v e n a r i g h t w i t h b a s e = 24 a n d h e i g h t = 18 Given \space a \space right \space \triangle \space with \space base \space= \space 24 \space and \space height \space= \space 18

A f t e r r e v o l v i n g t h e a l o n g e i t h e r o f i t s a r m s , w e w i l l o b t a i n a C O N E After \space revolving \space the \space \triangle \space along \space either \space of \space its \space arms \space , \space we \space will \space obtain \space a \space \boxed{CONE}

C a s e 1 : Case \space 1 \space:

R a d i u s o f c o n e = h e i g h t o f = 18 Radius \space of \space cone \space = \space height \space of \space \triangle \space= \space 18

H e i g h t o f c o n e = b a s e o f = 24 Height \space of \space cone \space = \space base \space of \space \triangle \space= \space 24

V o l u m e ( V 1 ) = 1 3 π ( 18 ) 2 × 24 \therefore \space Volume \space (V_{1}) \space=\space \frac{1}{3}\space\pi (18)^{2} \times 24

C a s e 2 : Case \space 2 \space:

R a d i u s o f c o n e = b a s e o f = 24 Radius \space of \space cone \space = \space base \space of \space \triangle \space= \space 24

H e i g h t o f c o n e = h e i g h t o f = 18 Height \space of \space cone \space = \space height \space of \space \triangle \space= \space 18

V o l u m e ( V 2 ) = 1 3 π ( 24 ) 2 × 18 \therefore \space Volume \space (V_{2}) \space=\space \frac{1}{3}\space\pi (24)^{2} \times 18

V o l u m e R a t i o = V 1 V 2 = 1 3 π ( 18 ) 2 × 24 1 3 π ( 24 ) 2 × 18 = 3 4 \Rightarrow \space Volume \space Ratio \space = \space \frac{V_{1}}{V_{2}} \space=\space \frac{ \frac{1}{3}\space\pi (18)^{2} \times 24}{\frac{1}{3}\space\pi (24)^{2} \times 18} \space = \space \frac{3}{4}

V o l u m e R a t i o = 3 : 4 \therefore \space \boxed{Volume \space Ratio \space = \space 3 \space : \space 4}

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