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If y = cosx+cosx+cosx+......∞, show that dydx=sinx1-2y - Mathematics and Statistics

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Question

If y = `sqrt(cos x + sqrt(cos x + sqrt(cos x + ...... ∞)`, show that `("d"y)/("d"x) = (sin x)/(1 - 2y)`

Sum
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Solution

y = `sqrt(cos x + sqrt(cos x + sqrt(cos x + ...  ∞)`

∴ y2 = `cos x + sqrt(cos x + sqrt(cos x + ...  ∞)`

∴ y2 = cos x + y

Differentiating w. r. t. x, we get

`2y ("d"y)/("d"x) = -sin x + ("d"y)/("d"x)`

∴ `("d"y)/("d"x)(1 - 2y)` = sin x

∴ `("d"y)/("d"x) = (sin x)/(1 - 2y)`

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Chapter 2.1: Differentiation - Short Answers II

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