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MA Afroz

· started a discussion

· 1 Months ago

Vol of frustum=total vol - vol of smaller cone.
Above way is simple rather than the one which provided.

Question:

If a cone is cut into two parts by a horizontal plane passing through the mid point of its axis, the ratio of the volumes of the upper part and the frustum is:

Options:
A) 1 : 1 
B) 1 : 2  
C) 1 : 3 
D) 1 : 7
Solution:




Knowledge Expert

· commented

· 1 Months ago

Dear Student,

Yes you are right. we can solved it another way.

its volume V = (1/3)(π)(R²)(H) cu. units = (πR²H)/3

2) As the horizontal plane cuts the cone into two parts through the mid point of its axis, the height of the cone is divided into two equal parts, forming a top cone of height H/2 units. If the base radius of the top cone is r units, then r/(H/2) = R/H [Since the vertical angle of both are same].

Solving, r = R/2 units.

Hence volume of the small cone at the top = v = (1/3)(π){(R/2)²{(H/2) cu. units
Simplifying, v = (πR²H)/24 cu. units

So volume of the bottom part (frustum) = V - v = (πR²H)/3 - (πR²H)/24 = 7(πR²H)/24 cu. units

3) So ratio of their volumes, Top part : Bottom part = (πR²H)/24 : 7(πR²H)/24 = 1:7

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