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If "X"^(5/3) . "Y"^(2/3) = ("X + Y")^(7/3) , the Show that "Dy"/"Dx" = "Y"/"X" - Mathematics and Statistics

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Question

If `"x"^(5/3) . "y"^(2/3) = ("x + y")^(7/3)` , the show that `"dy"/"dx" = "y"/"x"`

Sum

Solution

`"x"^(5/3) . "y"^(2/3) = ("x + y")^(7/3)`

Taking logarithm of both the sides , we get

`5/3 "log x" + 2/3 "log y" = 7/3 "log (x + y)"`

Differentiating both sides w.r.t. x, 


`5/3 . 1/"x" + 2/3 . 1/"y" "dy"/"dx" = 7/3 . 1/("x + y") [1 + "dy"/"dx"]`


`therefore 2/"3y" "dy"/"dx" - 7/(3 ("x + y"))  "dy"/"dx" = 7/(3 ("x + y")) - 5/"3x"`


`therefore [2/"3y" - 7/(3 ("x + y"))] "dy"/"dx" = (7"x" - 5("x + y"))/("3x"("x + y"))`


`therefore [(2("x + y") - "7y")/("3y" ("x + y"))] "dy"/"dx" = (7"x" - 5("x + y"))/("3x"("x + y"))`


`therefore ("2x" - "5y")/"y" "dy"/"dx" = (2"x" - 5"y")/"x"`


`therefore "dy"/"dx" = "y"/"x"`

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2014-2015 (October)

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