Prove that the semi-vertical angle of …
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Gaurav Seth 4 years, 2 months ago
Solution:
Let V be the given volume of cone.
V=Volume of cone =
h = Height of cone =
C=Curved surface area of cone =
, where r is radius and l is slant height of cone.
C=
, as l²= r²+ h²
For Maxima and Minima, derivative of C that is curved surface area should be equal to zero.
K=C²= π²r²(r²+h²)
K =%5E2%20r%5E4%7D%3D%5Cpi%20%7D%5E2%20r%20%5E4%20%2B%5Cfrac%7B9V%5E2%7D%7B%20r%5E2%20%7D)
Differentiating both sides with respect to r
K' = 4π²r³ +
Putting , K'=0
h=%5E2%7D%5D%5E%5Cfrac%7B1%7D%7B3%7D%7D%20%3D(%5Cfrac%7B6%20V%7D%7B%5Cpi%7D)%5E%5Cfrac%7B1%7D%7B3%7D)
Let A be the semi vertical angle of the cone.
Cot A =
=%5E%5Cfrac%7B1%7D%7B3%7D%7D%7B%7B%5Cfrac%7B3%5E%5Cfrac%7B1%7D%7B3%7D%20V%5E%5Cfrac%7B1%7D%7B3%7D%7D%7B2%5E%5Cfrac%7B1%7D%7B6%7D(%5Cpi)%5E%7B%5Cfrac%7B1%7D%7B3%7D%7D%7D%7D%7D)
Cot A=
A=
Hence proved.
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