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Find dydxdydx, if x = uyu1+u2,y=log(1+u2) - Mathematics and Statistics

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Question

Find `"dy"/"dx"`, if x = `sqrt(1 + "u"^2), "y" = log (1 + "u"^2)`

Sum
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Solution

x = `sqrt(1 + "u"^2)`

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

`"dx"/"du" = "d"/"du"(sqrt(1 + "u"^2))`

= `1/(2sqrt(1 + "u"^2)) * "d"/"dx" (1 + "u"^2)`

= `1/(2sqrt(1 + "u"^2)) xx 2"u"`

= `"u"/sqrt(1 + "u"^2)`

y = log (1 + u2)

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

`"dy"/"du" = "d"/"dx"[log (1 + "u"^2)]`

= `1/(1 + "u"^2) * "d"/"du" (1 + "u"^2)`

= `1/(1 + "u"^2) xx "2u"`

= `"2u"/(1 + "u"^2)`

∴ `"dy"/"dx" = (("dy"/"du"))/(("dx"/"du"))`

= `(("2u"/(1  +  "u"^2)))/(("u"/sqrt(1  +  "u"^2)))`

= `2/(1 + "u"^2) xx sqrt(1 + "u"^2)/"u"`

= `(2sqrt(1 + "u"^2))/(1+u^2)`

= `(2sqrt(1 + "u"^2))/(sqrt(1+u^2)xxsqrt(1+u^2))`

∴ `"dy"/"dx" = 2/sqrt(1 + "u"^2)`

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Derivatives of Parametric Functions
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Chapter 3: Differentiation - EXERCISE 3.5 [Page 97]

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