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If ex + ey = ex+y, then show that dy/dx = -e^(y - x). - Mathematics and Statistics

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Question

If ex + ey = ex+y, then show that `"dy"/"dx" = -e^(y - x)`.

Sum
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Solution

ex + ey = ex+y                               ...[1]

Differentiating both sides w.r.t. x, we get

`e^x + e^y."dy"/"dx" = e^(x + y)."d"/"dx"(x + y)`

∴ `e^x + e^y."dy"/"dx" = e^(x + y).(1 + "dy"/"dx")`

∴ `e^x + e^y"dy"/"dx" = e^(x + y) + e^(x + y)"dy"/"dx"`

∴ `(e^y - e^(x + y))"dy"/"dx" = e^(x + y) –e^x`

∴ `"dy"/"dx" = (e^(x +y) - e^x)/(e^y - e^(x + y)`

= `((e^x + e^y) - e^x)/(e^y - (e^x + e^y))`    ...[Using (1), since `e^(x + y) = e^x + e^y`]

= `e^y/-e^x`       

= – ey – x

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Chapter 1: Differentiation - Exercise 1.3 [Page 40]

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