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Application of Derivatives question

2014 · Shift 1 · Q27
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Application of Derivatives question

2014 · Shift 1 · Q27

JEE AdvancedMathematicsApplication of DerivativesNumerical+3 / −1
The slope of the tangent to the curve (y−x5)2=x(1+x2)2{\left( {y - {x^5}} \right)^2} = x{\left( {1 + {x^2}} \right)^2}(y−x5)2=x(1+x2)2 at the point (1,3)(1, 3)(1,3) is
Numerical answer
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Correct answer: 8

The problem asks for the slope of the tangent to the curve (y−x5)2=x(1+x2)2{\left( {y - {x^5}} \right)^2} = x{\left( {1 + {x^2}} \right)^2}(y−x5)2=x(1+x2)2 at the point (1,3)(1, 3)(1,3). The slope of the tangent is given by the value of the derivative, dydx\frac{dy}{dx}dxdy​, at the given point.

Step 1: Verify the point is on the curve

First, let's check if the point (1,3)(1, 3)(1,3) lies on the given curve by substituting x=1x=1x=1 and y=3y=3y=3 into the equation.

Left Hand Side (LHS): (y−x5)2=(3−15)2=(3−1)2=22=4{\left( {y - {x^5}} \right)^2} = {\left( {3 - {1^5}} \right)^2} = {\left( {3 - 1} \right)^2} = {2^2} = 4(y−x5)2=(3−15)2=(3−1)2=22=4 Right Hand Side (RHS): x(1+x2)2=1(1+12)2=1(1+1)2=22=4x{\left( {1 + {x^2}} \right)^2} = 1{\left( {1 + {1^2}} \right)^2} = 1{\left( {1 + 1} \right)^2} = {2^2} = 4x(1+x2)2=1(1+12)2=1(1+1)2=22=4 Since LHS = RHS, the point (1,3)(1, 3)(1,3) is indeed on the curve.

Step 2: Differentiate the equation with respect to x

We will use implicit differentiation to find dydx\frac{dy}{dx}dxdy​. The given equation is: (y−x5)2=x(1+x2)2{\left( {y - {x^5}} \right)^2} = x{\left( {1 + {x^2}} \right)^2}(y−x5)2=x(1+x2)2

Differentiating the LHS using the chain rule: ddx(y−x5)2=2(y−x5)⋅ddx(y−x5)\frac{d}{dx} {\left( {y - {x^5}} \right)^2} = 2(y - x^5) \cdot \frac{d}{dx}(y - x^5)dxd​(y−x5)2=2(y−x5)⋅dxd​(y−x5) =2(y−x5)(dydx−5x4) = 2(y - x^5) \left( \frac{dy}{dx} - 5x^4 \right)=2(y−x5)(dxdy​−5x4)

Differentiating the RHS using the product rule and chain rule: Let u=xu = xu=x and v=(1+x2)2v = (1 + x^2)^2v=(1+x2)2. Then dudx=1\frac{du}{dx} = 1dxdu​=1. dvdx=2(1+x2)⋅ddx(1+x2)=2(1+x2)(2x)=4x(1+x2)\frac{dv}{dx} = 2(1 + x^2) \cdot \frac{d}{dx}(1 + x^2) = 2(1 + x^2)(2x) = 4x(1 + x^2)dxdv​=2(1+x2)⋅dxd​(1+x2)=2(1+x2)(2x)=4x(1+x2) So, the derivative of the RHS is: ddx[x(1+x2)2]=dudxv+udvdx\frac{d}{dx} \left[ x{\left( {1 + {x^2}} \right)^2} \right] = \frac{du}{dx}v + u\frac{dv}{dx}dxd​[x(1+x2)2]=dxdu​v+udxdv​ =1⋅(1+x2)2+x⋅[4x(1+x2)]= 1 \cdot (1 + x^2)^2 + x \cdot [4x(1 + x^2)]=1⋅(1+x2)2+x⋅[4x(1+x2)] =(1+x2)2+4x2(1+x2)= (1 + x^2)^2 + 4x^2(1 + x^2)=(1+x2)2+4x2(1+x2) Factoring out (1+x2)(1 + x^2)(1+x2): =(1+x2)[(1+x2)+4x2]=(1+x2)(1+5x2)= (1 + x^2)[(1 + x^2) + 4x^2] = (1 + x^2)(1 + 5x^2)=(1+x2)[(1+x2)+4x2]=(1+x2)(1+5x2)

Step 3: Equate the derivatives

Now, we equate the derivatives of the LHS and RHS: 2(y−x5)(dydx−5x4)=(1+x2)(1+5x2)2(y - x^5) \left( \frac{dy}{dx} - 5x^4 \right) = (1 + x^2)(1 + 5x^2)2(y−x5)(dxdy​−5x4)=(1+x2)(1+5x2)

Step 4: Substitute the point (1, 3) and solve for dy/dx

Substitute x=1x=1x=1 and y=3y=3y=3 into the differentiated equation: 2(3−15)(dydx−5(1)4)=(1+12)(1+5(1)2)2(3 - 1^5) \left( \frac{dy}{dx} - 5(1)^4 \right) = (1 + 1^2)(1 + 5(1)^2)2(3−15)(dxdy​−5(1)4)=(1+12)(1+5(1)2) 2(3−1)(dydx−5)=(1+1)(1+5)2(3 - 1) \left( \frac{dy}{dx} - 5 \right) = (1 + 1)(1 + 5)2(3−1)(dxdy​−5)=(1+1)(1+5) 2(2)(dydx−5)=(2)(6)2(2) \left( \frac{dy}{dx} - 5 \right) = (2)(6)2(2)(dxdy​−5)=(2)(6) 4(dydx−5)=124 \left( \frac{dy}{dx} - 5 \right) = 124(dxdy​−5)=12

Now, we solve for dydx\frac{dy}{dx}dxdy​: dydx−5=124\frac{dy}{dx} - 5 = \frac{12}{4}dxdy​−5=412​ dydx−5=3\frac{dy}{dx} - 5 = 3dxdy​−5=3 dydx=3+5\frac{dy}{dx} = 3 + 5dxdy​=3+5 dydx=8\frac{dy}{dx} = 8dxdy​=8

Thus, the slope of the tangent to the curve at the point (1,3)(1, 3)(1,3) is 8.

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