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If x = cos (2t) and y = sin2 t, then what is \(\rm \dfrac{d^2 y}{dx^2}\) equal to?
5. None of these

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Correct Answer - Option 1 : 0

Concept:

Let x =  f(t) and y = f(t) then 

By Chain Rule, we have
\(\rm \dfrac{d y}{dx} = \dfrac{\dfrac {dy}{dt}}{\dfrac {dx}{dt}}\)

sin (2t) = 2 sin t cos t

Calculations:

Given, x = cos (2t) 

\(\rm \dfrac {dx}{dt} = - 2\;sin \;(2t)\)

and y = sin2 t 

⇒ \(\rm \dfrac {dy}{dt} = 2\;sin \;tcos\;t\)

\(\rm \dfrac {dy}{dt} = sin \;2t\)

By Chain Rule, we have

\(\rm \dfrac{d y}{dx} = \dfrac{\dfrac {dy}{dt}}{\dfrac {dx}{dt}}\)

\(\rm \dfrac{d y}{dx} =\dfrac {sin \; 2t}{-2sin \;2t}\)

\(\rm \dfrac{d y}{dx} =\dfrac {-1}{2}\)

Differentiating with respect to x, we get

\(\rm \dfrac{d^2 y}{dx^2} = 0\)

Hence, if  x = cos (2t) and y = sin2 t, then  \(\rm \dfrac{d^2 y}{dx^2}\) equal to 0

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