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If y = (cosx)(cosx)(cosx)....∞, show that dydxdydx=y2tanxylogcosx-1

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प्रश्न

If y = `(cos x)^((cos x)^((cosx)....oo)`, show that `"dy"/"dx" = (y^2 tanx)/(y log cos x - 1)`

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उत्तर

Given that y = `(cos x)^((cos x)^((cosx)....oo)`,

⇒ y = (cos x)y  .....`[y = (cos x)^((cos x)^((cosx)....oo))]`

Taking log on both sides log y = y.log(cos x)

Differentiating both sides w.r.t. x

`1/y * "dy"/"dx" = y * "d"/"dx" log (cos x ) + log(cos x) * "dy"/"dx"`

⇒ `1/y * "dy"/"dx" = y * 1/cosx * "d"/"dx" (cos x) + log(cos x) * "dy"/"dx"`

⇒ `1/y * "dy"/"dx" = y* 1/cosx * (- sin x) + log(cosx) * "dy"/"dx"`

⇒ `1/y * "dy"/"dx" - log(cos x) "dy"/"dx"` = – y tan x

⇒ `[1/y - log (cosx)] "dy"/"dx"` = – y tan x

⇒ `"dy"/"dx" = (- y tanx)/(1/y - log(cosx))`

= `(y^2 tanx)/(y log cos x - 1)`

Hence, `"dy"/"dx" = (y^2 tanx)/(y log cos x - 1)`.

Hence proved.

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अध्याय 5: Continuity And Differentiability - Exercise [पृष्ठ १११]

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एनसीईआरटी एक्झांप्लर Mathematics Exemplar [English] Class 12
अध्याय 5 Continuity And Differentiability
Exercise | Q 61 | पृष्ठ १११

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