Maths-
General
Easy

Question

A conical vessel of height 10ft and semivertical angle 30 to the power of ring operator end exponent is full of water. If empties in such away that the height at the water in the vessel is decreasing at a constant rate of 1ft/min. Then the rate at which the volume of water in the vessel is decreasing when its height is  6ft. is

  1. pi    
  2. 12 pi    
  3. fraction numerator pi over denominator 2 end fraction    
  4. 3 pi    

hintHint:

Rate of change of volume with respect to time ( fraction numerator d v over denominator d t end fraction) = 1 third straight pi fraction numerator d over denominator d t end fraction left parenthesis r squared straight h right parenthesis

The correct answer is: 12 pi



    Given : 
    theta space equals space 30 degree
h space equals space 10 f t
fraction numerator d h over denominator d t end fraction space equals space 1 f t divided by m i n
r a t e space o f space c h a n g e space left parenthesis h right parenthesis space equals space 6 f t
    We know that, tantheta = fraction numerator o p p o s i t e space s i d e over denominator a d j a c e n t space s i d e end fraction
    tantheta = r over h
    tan 30 degree space equals space r over h
tan 30 degree space equals space fraction numerator 1 over denominator square root of 3 end fraction
r space equals space fraction numerator h over denominator square root of 3 end fraction
    Volume of conical vessel (V) = 1 third πr squared straight h
    V = 1 third straight pi straight h squared over 3 straight h space equals space 1 over 9 πh cubed
    Differentiating the above wrt to time
    fraction numerator d v over denominator d t end fraction space equals space 1 over 9 straight pi straight d over dt left parenthesis straight h cubed right parenthesis
straight d over dt open parentheses straight x to the power of straight n close parentheses space equals space straight n space. space straight x to the power of straight n minus 1 end exponent space cross times space dx
    fraction numerator d v over denominator d t end fraction space equals space straight pi over 9 cross times 3 h squared space cross times fraction numerator d h over denominator d t end fraction
S u b s t i t u t i n g space t h e space v a l u e s space o f space h space a n d space fraction numerator d h over denominator d t end fraction
fraction numerator d v over denominator d t end fraction space equals space space straight pi over 9 cross times space 3 space cross times left parenthesis 6 right parenthesis squared cross times 1
fraction numerator d v over denominator d t end fraction space equals space 12 straight pi
    Thus, the rate at which the volume of water in the vessel is decreasing when its height is  6ft. is 12 straight pi

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