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JEE Advanced 2014 · Application of Derivatives

The slope of the tangent to the curve (y−x5)2=x(1+x2)2(y - x^{5})^{2} = x(1 + x^{2})^{2} at the point (1,3)(1, 3) is

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Written solution

The slope of the tangent to the curve (y - x⁵)² = x(1 + x²)² at the point (1, 3) is

(y−x5)2=x(1+x2)2(y-x^5)^2 = x(1+x^2)^2

point (1,3)\text{point } (1,3)

slope=dydx∣(1,3)\text{slope} = \left.\dfrac{dy}{dx}\right|_{(1,3)}

1. differentiate both sides in x1.\ \text{differentiate both sides in } x

2. simplify the right side2.\ \text{simplify the right side}

3. put (1,3), solve for y′3.\ \text{put } (1,3),\ \text{solve for } y'

2(y−x5)(y′−5x4)=ddx[x(1+x2)2]2(y-x^5)(y'-5x^4) = \dfrac{d}{dx}\left[x(1+x^2)^2\right]

=(1+x2)2+x⋅2(1+x2)(2x)= (1+x^2)^2 + x\cdot 2(1+x^2)(2x)

=(1+x2)[(1+x2)+4x2]=(1+x2)(1+5x2)= (1+x^2)\left[(1+x^2)+4x^2\right] = (1+x^2)(1+5x^2)

at (1,3): y−x5=2,(1+x2)(1+5x2)=2⋅6=12\text{at } (1,3):\ y-x^5 = 2,\quad (1+x^2)(1+5x^2) = 2\cdot 6 = 12

2⋅2 (y′−5)=122\cdot 2\,(y'-5) = 12

4(y′−5)=12⇒y′−5=34(y'-5) = 12 \Rightarrow y'-5 = 3

y′=8y' = 8

slope=8\text{slope} = 8

8\boxed{8}

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