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Limits Continuity and Differentiability question

2022 · Shift 2 · Q23
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  5. /2022 · Shift 2 · Q23

Limits Continuity and Differentiability question

2022 · Shift 2 · Q23

JEE AdvancedMathematicsLimits Continuity and DifferentiabilityNumerical+3 / −1
If β=lim⁡x→0ex3−(1−x3)13+((1−x2)12−1)sin⁡xxsin⁡2x,\beta=\lim \limits_{x \to 0} \frac{e^{x^{3}}-\left(1-x^{3}\right)^{\frac{1}{3}}+\left(\left(1-x^{2}\right)^{\frac{1}{2}}-1\right) \sin x}{x \sin ^{2} x},β=x→0lim​xsin2xex3−(1−x3)31​+((1−x2)21​−1)sinx​, then the value of 6β6 \beta6β is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 5

  1. We need to evaluate
β=lim⁡x→0ex3−(1−x3)1/3+((1−x2)1/2−1)sin⁡xxsin⁡2x.\beta=\lim_{x\to 0}\frac{e^{x^3}-\left(1-x^3\right)^{1/3}+\left(\left(1-x^2\right)^{1/2}-1\right)\sin x}{x\sin^2 x}.β=x→0lim​xsin2xex3−(1−x3)1/3+((1−x2)1/2−1)sinx​.
  1. Since x→0x\to 0x→0, use series expansions.

Step 1: Expand each term

(i) Expansion of ex3e^{x^3}ex3

ex3=1+x3+x62+⋯e^{x^3}=1+x^3+\frac{x^6}{2}+\cdotsex3=1+x3+2x6​+⋯

(ii) Expansion of (1−x3)1/3(1-x^3)^{1/3}(1−x3)1/3

Using

(1+u)α=1+αu+α(α−1)2u2+⋯(1+u)^\alpha=1+\alpha u+\frac{\alpha(\alpha-1)}{2}u^2+\cdots(1+u)α=1+αu+2α(α−1)​u2+⋯

with u=−x3u=-x^3u=−x3, α=13\alpha=\frac13α=31​,

(1−x3)1/3=1−x33−x69+⋯(1-x^3)^{1/3}=1-\frac{x^3}{3}-\frac{x^6}{9}+\cdots(1−x3)1/3=1−3x3​−9x6​+⋯

Therefore,

ex3−(1−x3)1/3=(1+x3+x62+⋯ )−(1−x33−x69+⋯ )e^{x^3}-(1-x^3)^{1/3} =\left(1+x^3+\frac{x^6}{2}+\cdots\right)-\left(1-\frac{x^3}{3}-\frac{x^6}{9}+\cdots\right)ex3−(1−x3)1/3=(1+x3+2x6​+⋯)−(1−3x3​−9x6​+⋯) =43x3+1118x6+⋯=\frac{4}{3}x^3+\frac{11}{18}x^6+\cdots=34​x3+1811​x6+⋯

(iii) Expansion of (1−x2)1/2−1(1-x^2)^{1/2}-1(1−x2)1/2−1

Using

(1−x2)1/2=1−x22−x48+⋯(1-x^2)^{1/2}=1-\frac{x^2}{2}-\frac{x^4}{8}+\cdots(1−x2)1/2=1−2x2​−8x4​+⋯

so

(1−x2)1/2−1=−x22−x48+⋯(1-x^2)^{1/2}-1=-\frac{x^2}{2}-\frac{x^4}{8}+\cdots(1−x2)1/2−1=−2x2​−8x4​+⋯

Also,

sin⁡x=x−x36+⋯\sin x=x-\frac{x^3}{6}+\cdotssinx=x−6x3​+⋯

Hence

((1−x2)1/2−1)sin⁡x=(−x22−x48+⋯ )(x−x36+⋯ ).\left((1-x^2)^{1/2}-1\right)\sin x =\left(-\frac{x^2}{2}-\frac{x^4}{8}+\cdots\right)\left(x-\frac{x^3}{6}+\cdots\right).((1−x2)1/2−1)sinx=(−2x2​−8x4​+⋯)(x−6x3​+⋯).

The lowest-order term is

−x32+⋯-\frac{x^3}{2}+\cdots−2x3​+⋯

So the numerator becomes

43x3−12x3+⋯\frac{4}{3}x^3-\frac{1}{2}x^3+\cdots34​x3−21​x3+⋯ =(86−36)x3+⋯=56x3+⋯=\left(\frac{8}{6}-\frac{3}{6}\right)x^3+\cdots=\frac{5}{6}x^3+\cdots=(68​−63​)x3+⋯=65​x3+⋯
  1. Now expand the denominator:
xsin⁡2x=x(x−x36+⋯ )2.x\sin^2 x=x\left(x-\frac{x^3}{6}+\cdots\right)^2.xsin2x=x(x−6x3​+⋯)2.

Since

sin⁡2x=x2+O(x4),\sin^2 x=x^2+O(x^4),sin2x=x2+O(x4),

we get

xsin⁡2x=x3+O(x5).x\sin^2 x=x^3+O(x^5).xsin2x=x3+O(x5).
  1. Therefore,
β=lim⁡x→056x3+⋯x3+⋯=56.\beta=\lim_{x\to 0}\frac{\frac{5}{6}x^3+\cdots}{x^3+\cdots}=\frac{5}{6}.β=x→0lim​x3+⋯65​x3+⋯​=65​.

Hence,

6β=6⋅56=5.6\beta=6\cdot \frac{5}{6}=5.6β=6⋅65​=5.

Final Answer

5\boxed{5}5​

The derived answer matches the stored correct answer.

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