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Application of Derivatives question

2010 · Shift 2 · Q22
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Application of Derivatives question

2010 · Shift 2 · Q22

JEE AdvancedMathematicsApplication of DerivativesNumerical+4 / −1
Let fff be a function defined on RRR(the set of all real numbers) such that f′(x)=2010(x−2009)(x−2010)2(x−2011)3(x−2012)4f'\left( x \right) = 2010\left( {x - 2009} \right){\left( {x - 2010} \right)^2}{\left( {x - 2011} \right)^3}{\left( {x - 2012} \right)^4}f′(x)=2010(x−2009)(x−2010)2(x−2011)3(x−2012)4 for all x∈Rx \in Rx∈R If ggg is a function defined on RRR with values in the interval (0,∞)\left( {0,\infty } \right)(0,∞) such that f(x)=ln (g(x)),  for  all  x∈Rf\left( x \right) = ln\,\left( {g\left( x \right)} \right),\,\,for\,\,all\,\,x \in Rf(x)=ln(g(x)),forallx∈R then the number of points in RRR at which ggg has a local maximum is ‾\underline{\hspace{2cm}}​.
Numerical answer
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Correct answer: 1

  1. We are given f′(x)=2010(x−2009)(x−2010)2(x−2011)3(x−2012)4.f'(x)=2010(x-2009)(x-2010)^2(x-2011)^3(x-2012)^4.f′(x)=2010(x−2009)(x−2010)2(x−2011)3(x−2012)4. Also, f(x)=ln⁡(g(x)),f(x)=\ln(g(x)),f(x)=ln(g(x)), with g(x)>0g(x)>0g(x)>0 for all x∈Rx\in \mathbb Rx∈R.

  2. Since f(x)=ln⁡(g(x)),f(x)=\ln(g(x)),f(x)=ln(g(x)), and ln⁡x\ln xlnx is a strictly increasing function on (0,∞)(0,\infty)(0,∞), the local maxima/minima of fff and ggg occur at the same points.

Indeed, f′(x)=g′(x)g(x).f'(x)=\frac{g'(x)}{g(x)}.f′(x)=g(x)g′(x)​. Since g(x)>0g(x)>0g(x)>0, f′(x)f'(x)f′(x) and g′(x)g'(x)g′(x) always have the same sign. Hence the intervals of increase/decrease of fff and ggg are identical.

So it is enough to count the number of local maxima of fff.

  1. Critical points come from f′(x)=0f'(x)=0f′(x)=0: x=2009,  2010,  2011,  2012.x=2009,\;2010,\;2011,\;2012.x=2009,2010,2011,2012. Now analyze sign changes of f′(x)f'(x)f′(x).

Because the constant 2010>02010>02010>0, the sign depends on (x−2009)(x−2010)2(x−2011)3(x−2012)4.(x-2009)(x-2010)^2(x-2011)^3(x-2012)^4.(x−2009)(x−2010)2(x−2011)3(x−2012)4.

Notice:

  • (x−2010)2(x-2010)^2(x−2010)2 is always nonnegative and has even multiplicity, so it does not change sign across x=2010x=2010x=2010.
  • (x−2012)4(x-2012)^4(x−2012)4 is always nonnegative and has even multiplicity, so it does not change sign across x=2012x=2012x=2012.
  • (x−2009)(x-2009)(x−2009) changes sign across x=2009x=2009x=2009.
  • (x−2011)3(x-2011)^3(x−2011)3 changes sign across x=2011x=2011x=2011.

Thus the sign of f′(x)f'(x)f′(x) is essentially the sign of (x−2009)(x−2011).(x-2009)(x-2011).(x−2009)(x−2011).

  1. Check intervals:
  • For x<2009x<2009x<2009: x−2009<0,x−2011<0  ⟹  f′(x)>0.x-2009<0,\quad x-2011<0 \implies f'(x)>0.x−2009<0,x−2011<0⟹f′(x)>0.

  • For 2009<x<20102009<x<20102009<x<2010: x−2009>0,x−2011<0  ⟹  f′(x)<0.x-2009>0,\quad x-2011<0 \implies f'(x)<0.x−2009>0,x−2011<0⟹f′(x)<0.

  • For 2010<x<20112010<x<20112010<x<2011: again x−2009>0,x−2011<0  ⟹  f′(x)<0.x-2009>0,\quad x-2011<0 \implies f'(x)<0.x−2009>0,x−2011<0⟹f′(x)<0.

  • For 2011<x<20122011<x<20122011<x<2012: x−2009>0,x−2011>0  ⟹  f′(x)>0.x-2009>0,\quad x-2011>0 \implies f'(x)>0.x−2009>0,x−2011>0⟹f′(x)>0.

  • For x>2012x>2012x>2012: x−2009>0,x−2011>0  ⟹  f′(x)>0.x-2009>0,\quad x-2011>0 \implies f'(x)>0.x−2009>0,x−2011>0⟹f′(x)>0.

So the sign chart is: +o− at x=2009,+ o - \text{ at } x=2009,+o− at x=2009, no sign change at x=2010x=2010x=2010, −→+ at x=2011,-\to + \text{ at } x=2011,−→+ at x=2011, no sign change at x=2012x=2012x=2012.

  1. Therefore:
  • At x=2009x=2009x=2009, fff changes from increasing to decreasing, so fff has a local maximum.
  • At x=2011x=2011x=2011, fff has a local minimum.
  • At x=2010x=2010x=2010 and x=2012x=2012x=2012, there is no local extremum.

Hence ggg also has exactly one local maximum.

  1. Final answer: 1\boxed{1}1​
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