Maths-
General
Easy

Question

The point on the ellipse 16x2 + 9y2 = 400 where ordinate decrease at the same rate at which the abscissa increases is

  1. (9, 16)    
  2. open parentheses 3 comma fraction numerator 16 over denominator 3 end fraction close parentheses    
  3. open parentheses fraction numerator 16 over denominator 3 end fraction comma 3 close parentheses    
  4. open parentheses negative 3 fraction numerator 16 over denominator 3 end fraction close parentheses    

hintHint:

We are given the equation of ellipse. We are given that the rate of decrease of ordinante is same as the rate of increase of abscissa. Rate means change w.r.t the time. To find the rate of change of quantity, we take it's derivative w.r.t time. We have to find the point at which the rate is as given above.

The correct answer is: open parentheses 3 comma fraction numerator 16 over denominator 3 end fraction close parentheses


    The given equation of ellipse is 16x2 + 9y2 = 400
    Ordinate means distance of the point from x-axis. It is given by y coordinate.
    Abscissa means distance of the point from the y-axis. It is given by x coordinate.
    We are given that,negative fraction numerator d y over denominator d t end fraction equals space fraction numerator d x over denominator d t end fraction  .....(1)
    We will take the derivate of the equation of ellipse w.r.t time.
    16 x squared space plus space 9 y squared space equals space 400
16 left parenthesis 2 right parenthesis x space fraction numerator d x over denominator d t end fraction plus space 9 left parenthesis 2 right parenthesis y fraction numerator d y over denominator d t end fraction space equals space 0
32 x space fraction numerator d x over denominator d t end fraction plus space 18 y fraction numerator d y over denominator d t end fraction equals 0
32 x fraction numerator d x over denominator d t end fraction equals space minus 18 y fraction numerator d y over denominator d t end fraction
N o w comma space u sin g space t h e space e q u a t i o n space left parenthesis 1 right parenthesis space i n space t h e space a b o v e space e q u a t i o n
32 x space open parentheses negative fraction numerator d y over denominator d t end fraction close parentheses space equals 18 y open parentheses negative fraction numerator d y over denominator d t end fraction close parentheses
32 x space equals space 18 y
D i v i d i n g space b o t h space t h e space s i d e s space b y space 2 space w e space g e t comma
16 x space equals 9 y
space space space space x space equals 9 over 16 y
space space
    Now, to find the points we will substitute the value of x in the equation of ellipse.
    16 x squared space plus space 9 y squared space equals space 400
16 open parentheses 9 over 16 close parentheses squared y squared space plus space 9 y squared space equals space 400
space space space space 9 squared over 16 y squared space plus space 9 y squared space equals space 400
space space space space space T a k i n g space 9 y squared space c o m m o n space
9 y squared open parentheses 9 over 16 space plus space 1 close parentheses space equals space 400
9 y squared open parentheses 25 over 16 close parentheses space equals space 400
M u l t i p l y i n g space b o t h space t h e space s i d e s space b y space open parentheses 16 over 25 close parentheses
9 y squared space equals space 400 space cross times space 16 over 25
9 y squared space equals space 16 space cross times space 16
y squared space equals space 256 over 9
    Taking square roots of both sides we get,
    y space equals plus-or-minus 16 over 3
    x space equals space 9 over 16 y
F o r space y space equals space plus 16 over 3
x space equals space 3
F o r space y space equals space minus 16 over 3
x space equals space minus 3
    So, the points satisfying the given condition are open parentheses 3 comma 16 over 3 close parentheses space a n d space open parentheses negative 3 comma negative 16 over 3 close parentheses
    This is the only point that matches the option open parentheses 3 comma 16 over 3 close parentheses.

    .

    For such questions, we should know how to find the derivates. We should know the concept of abscissa and ordinate.

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