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BITSAT 2021
Exam
BITSAT
Year
2021
Subject
Mathematics
Chapter
Application of Derivatives

BITSAT 2021 · Mathematics · Application of Derivatives

The points at which the tangent passes through the origin for the...

The points at which the tangent passes through the origin for the curve y=4x3−2x5y=4 x^{3}-2 x^{5} are

Options

  1. A(0,0),(2,1)(0,0),(2,1) and (−1,−2)(-1,-2)
  2. B(0,0),(2,1)(0,0),(2,1) and (−2,−1)(-2,-1)
  3. C(2,0),(2,1)(2,0),(2,1) and (−3,1)(-3,1)
  4. D(0,0),(1,2)(0,0),(1,2) and (−1,−2)(-1,-2)

Correct answer

D. (0,0),(1,2)(0,0),(1,2) and (−1,−2)(-1,-2)

Step-by-step solution

The equation of the given curve is y=4x3−2x5dydx=12x2−10x4\begin{array}{l} y=4 x^{3}-2 x^{5} \\ \frac{\mathrm{dy}}{\mathrm{dx}}=12 x^{2}-10 x^{4} \end{array} Therefore,theslopeofthetangentatpoint(x,y)the slope of the tangent at point(x, y) is 12x2−10x412 x^{2}-10 x^{4}. The equation of the tangent at (x,y)(x, y) is given by Y−y=(12x2−10x4)(X\mathrm{Y}-y=\left(12 x^{2}-10 x^{4}\right)(\mathrm{X}-x)… (i) x) \ldots \text{ (i) } When,thetangentpassesthroughtheorigin(0,0)the tangent passes through the origin(0,0), then X=Y=0Therefore,eq.(i)reduceto\mathrm{X}=\mathrm{Y}=0Therefore, eq. (i) reduce to−y=(12x2−10x4)(−x)⇒y=12x3−10x5\begin{array}{l} -y=\left(12 x^{2}-10 x^{4}\right)(-x) \\ \Rightarrow y=12 x^{3}-10 x^{5} \end{array}Also,wehaveAlso, we havey=4 x^{3}-2 x^{5}∴12x3−10x5=4x3−2x5⇒8x5−8x3=0⇒x5−x3=0⇒x3(x2−1)=0⇒x=0,±1\begin{array}{l} \therefore 12 x^{3}-10 x^{5} \\ =4 x^{3}-2 x^{5} \\ \Rightarrow 8 x^{5}-8 x^{3}=0 \\ \Rightarrow x^{5}-x^{3}=0 \\ \Rightarrow x^{3}\left(x^{2}-1\right)=0 \\ \Rightarrow x=0, \pm 1 \end{array} When, x=0y=4(0)3−2(0)5=0x=0y=4(0)^{3}-2(0)^{5}=0 When, x=1x=1, y=4(1)3−2(1)5=2y=4(1)^{3}-2(1)^{5}=2 When, x=−1x=-1, y=4(−1)3−2(−1)5=−2y=4(-1)^{3}-2(-1)^{5}=-2 Hence, the require points are (0,0),(1,2)and(−1,−2)(0,0),(1,2) and (-1,-2).
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