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Application of Derivatives question

2023 · 6 Apr · Shift 2 · Q44
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  5. /2023 · 6 Apr · Shift 2 · Q44

Application of Derivatives question

2023 · 6 Apr · Shift 2 · Q44

JEE MainMathematicsApplication of DerivativesNumerical+4 / −1
The number of points, where the curve y=x5−20x3+50x+2y=x^{5}-20 x^{3}+50 x+2y=x5−20x3+50x+2 crosses the x\mathrm{x}x-axis, is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 5

  1. We need the number of points where the curve y=x5−20x3+50x+2y=x^5-20x^3+50x+2y=x5−20x3+50x+2 crosses the xxx-axis.

    This means we need the number of real roots of f(x)=x5−20x3+50x+2=0.f(x)=x^5-20x^3+50x+2=0.f(x)=x5−20x3+50x+2=0.

  2. To determine how many times the graph can cross the xxx-axis, first study the monotonicity using the derivative.

    f′(x)=5x4−60x2+50=5(x4−12x2+10).f'(x)=5x^4-60x^2+50=5(x^4-12x^2+10).f′(x)=5x4−60x2+50=5(x4−12x2+10).

    Let u=x2u=x^2u=x2. Then x4−12x2+10=u2−12u+10=0.x^4-12x^2+10=u^2-12u+10=0.x4−12x2+10=u2−12u+10=0.

    Solving, u=6±26.u=6\pm\sqrt{26}.u=6±26​.

    Hence the critical points are at x=±6−26,±6+26.x=\pm\sqrt{6-\sqrt{26}},\quad \pm\sqrt{6+\sqrt{26}}.x=±6−26​​,±6+26​​.

  3. Let a=6−26,b=6+26.a=\sqrt{6-\sqrt{26}},\qquad b=\sqrt{6+\sqrt{26}}.a=6−26​​,b=6+26​​. Numerically,

    \qquad b\approx 3.33.$$ Since $$f'(x)=5(x^2-(6-\sqrt{26}))(x^2-(6+\sqrt{26})),$$ the sign of $f'(x)$ is: - positive for $|x|<a$ - negative for $a<|x|<b$ - positive for $|x|>b$ Therefore the function behaves as: - increasing on $(-\infty,-b)$ - decreasing on $(-b,-a)$ - increasing on $(-a,a)$ - decreasing on $(a,b)$ - increasing on $(b,\infty)$
  4. Now evaluate f(x)f(x)f(x) at convenient points around these intervals to track sign changes.

    Since fff is continuous, each sign change gives a root.

    Compute: f(−5)=(−5)5−20(−5)3+50(−5)+2=−3125+2500−250+2=−873<0f(-5)=(-5)^5-20(-5)^3+50(-5)+2=-3125+2500-250+2=-873<0f(−5)=(−5)5−20(−5)3+50(−5)+2=−3125+2500−250+2=−873<0 f(−4)=(−4)5−20(−4)3+50(−4)+2=−1024+1280−200+2=58>0f(-4)=(-4)^5-20(-4)^3+50(-4)+2=-1024+1280-200+2=58>0f(−4)=(−4)5−20(−4)3+50(−4)+2=−1024+1280−200+2=58>0

    So there is one root in (−5,−4)(-5,-4)(−5,−4).

    Next, f(−2)=(−32)−20(−8)−100+2=−32+160−100+2=30>0f(-2)=(-32)-20(-8)-100+2=-32+160-100+2=30>0f(−2)=(−32)−20(−8)−100+2=−32+160−100+2=30>0 f(−1)=−1+20−50+2=−29<0f(-1)=-1+20-50+2=-29<0f(−1)=−1+20−50+2=−29<0

    So there is one root in (−2,−1)(-2,-1)(−2,−1).

    Next, f(0)=2>0f(0)=2>0f(0)=2>0 so between −1-1−1 and 000, there is one root in (−1,0)(-1,0)(−1,0).

    Next, f(1)=1−20+50+2=33>0f(1)=1-20+50+2=33>0f(1)=1−20+50+2=33>0 f(2)=32−160+100+2=−26<0f(2)=32-160+100+2=-26<0f(2)=32−160+100+2=−26<0

    So there is one root in (1,2)(1,2)(1,2).

    Next, f(4)=1024−1280+200+2=−54<0f(4)=1024-1280+200+2=-54<0f(4)=1024−1280+200+2=−54<0 f(5)=3125−2500+250+2=877>0f(5)=3125-2500+250+2=877>0f(5)=3125−2500+250+2=877>0

    So there is one root in (4,5)(4,5)(4,5).

  5. Thus we have found 5 distinct real roots, in the intervals:

    \;(-2,-1), \;(-1,0), \;(1,2), \;(4,5).$$
  6. Since a quintic polynomial can have at most 5 real roots, these are all the roots.

Therefore, the number of points where the curve crosses the xxx-axis is 5.\boxed{5}.5​.

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