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Application Of Derivatives

Question
CBSEENMA12035217

Find points on the curve straight x squared over 9 plus straight y squared over 16 space equals space 1 at which the tangents are (i) parallel to the x-axis (ii) parallel to the y-axis.

Solution

The equation of curve is
                      straight x squared over 9 plus straight y squared over 16 space equals space 1                              ...(1)
Differentiating both sides w.r.t.x, we get,
                                          fraction numerator 2 straight x over denominator 9 end fraction plus fraction numerator 2 straight y over denominator 16 end fraction dy over dx space equals space 0 space space space space space or space space space space straight y over 8 dy over dx space equals space minus fraction numerator 2 straight x over denominator 9 end fraction
therefore space space space space space space space space space space space space space space dy over dx space equals space minus 16 over 9 straight x over straight y
(i) For the points on the curve,  where tangents are parallel to x-axis
                         dy over dx space equals space 0 space space space space space rightwards double arrow space space space space space minus fraction numerator 16 straight x over denominator 9 straight y end fraction space equals space 0 space space space space space rightwards double arrow space space space straight x space equals space 0
Putting x = 0 in (1), we get,
                                0 plus straight y squared over 16 space equals space 1 space space space space space space space space space space space rightwards double arrow space space space space space straight y squared space equals space 16 space space space space rightwards double arrow space space space straight y space equals space minus 4 comma space space space space 4
therefore space space space space space points space are space left parenthesis 0 comma space minus 4 right parenthesis comma space space left parenthesis 0 comma space 4 right parenthesis.
(ii) For the points on the curve,  where tangents are parallel to y-axis.
                   dx over dy space equals space 0 space space space space rightwards double arrow space space space minus fraction numerator 9 straight y over denominator 16 straight x end fraction space equals space 0 space space space space space rightwards double arrow space space space straight y space equals space 0
Putting y = 0 in (1), we get
                                 straight x squared over 9 plus 0 space equals space 1 space space space space rightwards double arrow space space space space space straight x squared space equals space 9 space space space space space rightwards double arrow space space space straight x space equals space minus 3 comma space space 3
therefore space space space space points space are space left parenthesis negative 3 comma space 0 right parenthesis comma space space left parenthesis 3 comma space 0 right parenthesis.