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Application of Integration question

2015 · Shift 2 · Q26
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  5. /2015 · Shift 2 · Q26

Application of Integration question

2015 · Shift 2 · Q26

JEE AdvancedMathematicsApplication of IntegrationNumerical+4 / −1
Let f:R→Rf:R \to Rf:R→R be a continuous odd function, which vanishes exactly at one point and f(1)=12.f\left( 1 \right) = {1 \over {2.}}f(1)=2.1​ Suppose that F(x)=∫−1xf(t)dtF\left( x \right) = \int\limits_{ - 1}^x {f\left( t \right)dt}F(x)=−1∫x​f(t)dt for all x∈  [−1,2]x \in \,\,\left[ { - 1,2} \right]x∈[−1,2] and G(x)=G(x)=G(x)=∫−1xt∣f(f(t))∣dt\int\limits_{ - 1}^x {t\left| {f\left( {f\left( t \right)} \right)} \right|} dt−1∫x​t∣f(f(t))∣dt for all x∈  [−1,2].x \in \,\,\left[ { - 1,2} \right].x∈[−1,2]. If lim⁡x→1F(x)G(x)=114,\mathop {\lim }\limits_{x \to 1} {{F\left( x \right)} \over {G\left( x \right)}} = {1 \over {14}},x→1lim​G(x)F(x)​=141​, then the value of f(12)f\left( {{1 \over 2}} \right)f(21​) is
Numerical answer
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Correct answer: 7

  1. Use the properties of an odd continuous function

Since f:R→Rf:\mathbb R\to\mathbb Rf:R→R is continuous and odd, f(−x)=−f(x).f(-x)=-f(x).f(−x)=−f(x). Also, it vanishes exactly at one point. For an odd continuous function, that point must be x=0x=0x=0. So:

  • f(0)=0f(0)=0f(0)=0,
  • f(x)≠0f(x)\neq 0f(x)=0 for x≠0x\neq 0x=0.

Given f(1)=12>0f(1)=\frac12>0f(1)=21​>0. By continuity, and since the only zero is at 000, we must have f(x)>0for x>0,f(x)<0for x<0.f(x)>0\quad \text{for }x>0, \qquad f(x)<0\quad \text{for }x<0.f(x)>0for x>0,f(x)<0for x<0.

Hence for t∈[−1,1)t\in[-1,1)t∈[−1,1), we have f(t)∈[−12,12)f(t)\in[-\tfrac12,\tfrac12)f(t)∈[−21​,21​), so in particular f(t)f(t)f(t) has the same sign as ttt.


  1. Compute F(1)F(1)F(1)

We have F(x)=∫−1xf(t) dt.F(x)=\int_{-1}^x f(t)\,dt.F(x)=∫−1x​f(t)dt. Thus F(1)=∫−11f(t) dt=0,F(1)=\int_{-1}^1 f(t)\,dt=0,F(1)=∫−11​f(t)dt=0, because the integral of an odd function over [−1,1][-1,1][−1,1] is 000.

Since lim⁡x→1F(x)G(x)=114\lim_{x\to 1}\frac{F(x)}{G(x)}=\frac1{14}limx→1​G(x)F(x)​=141​ is finite, we must also have G(1)=0G(1)=0G(1)=0.

Now G(1)=∫−11t ∣f(f(t))∣ dt.G(1)=\int_{-1}^1 t\,|f(f(t))|\,dt.G(1)=∫−11​t∣f(f(t))∣dt. Let h(t)=t ∣f(f(t))∣.h(t)=t\,|f(f(t))|.h(t)=t∣f(f(t))∣. Because fff is odd, f(f(t))f(f(t))f(f(t)) is also odd, hence ∣f(f(t))∣|f(f(t))|∣f(f(t))∣ is even. Therefore h(t)h(t)h(t) is odd. So indeed, G(1)=∫−11h(t) dt=0.G(1)=\int_{-1}^1 h(t)\,dt=0.G(1)=∫−11​h(t)dt=0. Thus the limit is of type 00\frac0000​, so we apply L'Hospital's Rule.


  1. Differentiate numerator and denominator

By the Fundamental Theorem of Calculus, F′(x)=f(x),G′(x)=x ∣f(f(x))∣.F'(x)=f(x), \qquad G'(x)=x\,|f(f(x))|.F′(x)=f(x),G′(x)=x∣f(f(x))∣. Therefore, lim⁡x→1F(x)G(x)=lim⁡x→1F′(x)G′(x)=lim⁡x→1f(x)x ∣f(f(x))∣.\lim_{x\to 1}\frac{F(x)}{G(x)}=\lim_{x\to 1}\frac{F'(x)}{G'(x)}=\lim_{x\to 1}\frac{f(x)}{x\,|f(f(x))|}.limx→1​G(x)F(x)​=limx→1​G′(x)F′(x)​=limx→1​x∣f(f(x))∣f(x)​.

Since fff is continuous and f(1)=12>0f(1)=\frac12>0f(1)=21​>0, we get \frac1{14}=\frac{f(1)}{1\cdot |f(f(1))|}= rac{\frac12}{|f(\frac12)|}. So, ∣f(12)∣=7.|f\left(\frac12\right)|=7.∣f(21​)∣=7.


  1. Determine the sign of f(12)f\left(\frac12\right)f(21​)

Because 12>0\frac12>021​>0 and f(x)>0f(x)>0f(x)>0 for all x>0x>0x>0, f(12)>0.f\left(\frac12\right)>0.f(21​)>0. Hence f(12)=7.f\left(\frac12\right)=7.f(21​)=7.


  1. Final answer

7\boxed{7}7​

This matches the stored correct answer.

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