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Application Of Derivatives

Question
CBSEENMA12035571

Find the equation of the line through the point (3, 4) which cuts from the first quadrant a triangle of minimum area.

Solution
The equation of any line through P (3, 4) is
   straight y minus 4 space equals space straight m left parenthesis straight x minus 3 right parenthesis space space space space space space space... left parenthesis 1 right parenthesis
where m is slope of line
   Put y = 0 in (1)
therefore space space space space space minus 4 space equals space mx space minus space 3 straight m space space space space space rightwards double arrow space space space mx space equals space 3 straight m minus 4
rightwards double arrow space space space space space space straight x space equals space 3 minus 4 over straight m
therefore space space space space space space line space left parenthesis 1 right parenthesis space meets space straight x minus axis
space space space space space space space space space space space space space in space straight A space open parentheses 3 minus 4 over straight m comma space 0 close parentheses
Again space put space straight x space equals space 0 space in space left parenthesis 1 right parenthesis
therefore space space space space space space space space straight y minus 4 space equals space minus 3 space straight m space space space rightwards double arrow space space space space straight y space equals space minus 3 straight m plus 4
therefore space space space space space line space left parenthesis 1 right parenthesis space meets space straight y minus axis space in space straight B left parenthesis 0 comma space minus 3 straight m space plus 4 right parenthesis

Let ∆ denote the area of ∆OAB
  therefore space space space space increment space equals space 1 half OA. space space OB space equals space 1 half open parentheses 3 minus 4 over straight m close parentheses space left parenthesis negative 3 straight m space plus space 4 right parenthesis
space space space space space space space space space space space space equals space 1 half open parentheses negative 9 straight m space plus 12 space plus space 12 space minus 16 over straight m close parentheses space equals space minus 9 over 2 straight m minus 8 over straight m
space space space space space fraction numerator straight d increment over denominator dm end fraction space equals space minus 9 over 2 plus 8 over straight m squared
space space space space space space space space fraction numerator straight d squared increment over denominator dm squared end fraction space equals space minus 16 over straight m cubed
For increment to be maximum or minimum,
          space fraction numerator straight d increment over denominator dm end fraction space equals space 0 space space space space or space space minus 9 over 2 plus 8 over straight m squared space equals space 0 space space space space space space rightwards double arrow space space space minus 9 straight m squared plus 16 space equals space 0
rightwards double arrow space space space straight m squared space equals space 16 over 9 space space space rightwards double arrow space space space space straight m space equals space minus 4 over 3 comma space space 4 over 3
When space straight m space equals space minus 4 over 3 comma space fraction numerator straight d squared increment over denominator dm squared end fraction greater than 0
therefore space space space space space space space increment space space space is space minimum space when space straight m space equals space minus 4 over 3
When space space space straight m space equals space 4 over 3 comma space space space fraction numerator straight d squared increment over denominator dm squared end fraction less than 0
therefore space space space space space space straight A space is space maximum space when space straight m space equals space 4 over 3
therefore space space space space space space space increment space space is space minimum space when space straight m space equals space minus 4 over 3
Putting space straight m space equals space minus 4 over 3 space in space left parenthesis 1 right parenthesis comma space we space get comma
space space space space space space space space space space straight y minus 4 space equals space minus 4 over 3 left parenthesis straight x minus 3 right parenthesis space space space space space space space space space or space space space space 3 straight y minus 12 space equals space minus 4 straight x plus 12
or space space space 4 straight x plus 3 straight y space equals space 24 comma
space
which is required equation of line.

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