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# Solution for If X = Cos T ( 3 − 2 Cos 2 T ) , Y = Sin T ( 3 − 2 Sin 2 T ) Find the Value of D Y D X a T T = π 4 ? - CBSE (Science) Class 12 - Mathematics

ConceptSimple Problems on Applications of Derivatives

#### Question

$\text { If }x = \cos t\left( 3 - 2 \cos^2 t \right), y = \sin t\left( 3 - 2 \sin^2 t \right) \text { find the value of } \frac{dy}{dx} at t = \frac{\pi}{4}$ ?

#### Solution

$x = \cos t\left( 3 - 2 \cos^2 t \right) \text { and y } = \sin t\left( 3 - 2 \sin^2 t \right)$
$\Rightarrow \frac{dx}{dt} = - \sin t\left( 3 - 2 \cos^2 t \right) + \cos t\left( 4\cos t\sin t \right) \text { and } \frac{dy}{dt} = \cos t\left( 3 - 2 \sin^2 t \right) + \sin t\left( - 4\sin t\cos t \right)$
$\Rightarrow \frac{dx}{dt} = - 3\sin t + 6\sin t \cos^2 t \text { and } \frac{dy}{dt} = 3\cos t - 6 \sin^2 t\cos t$
$\Rightarrow \frac{dx}{dt} = - 3\sin t\left( 1 - 2 \cos^2 t \right) \text { and } \frac{dy}{dt} = 3\cos t\left( 1 - 2 \sin^2 t \right)$
$\Rightarrow \frac{dx}{dt} = 3\sin t\cos2t \text { and } \frac{dy}{dt} = 3\cos t\cos2t$
$\therefore \frac{dy}{dx} = \frac{\frac{dy}{dt}}{\frac{dx}{dt}} = \frac{3\cos t\cos2t}{3\sin t\cos2t} = \cot t$
$\text { Now,} \left( \frac{dy}{dx} \right)_{t = \frac{\pi}{4}} = \cot\frac{\pi}{4} = 1$

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Solution If X = Cos T ( 3 − 2 Cos 2 T ) , Y = Sin T ( 3 − 2 Sin 2 T ) Find the Value of D Y D X a T T = π 4 ? Concept: Simple Problems on Applications of Derivatives.
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