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Limits Continuity and Differentiability question

2021 · 16 Mar · Shift 1 · Q41
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Limits Continuity and Differentiability question

2021 · 16 Mar · Shift 1 · Q41

JEE MainMathematicsLimits Continuity and DifferentiabilityNumerical+4 / −1
If lim⁡x→0aex−bcos⁡x+ce−xxsin⁡x=2\mathop {\lim }\limits_{x \to 0} {{a{e^x} - b\cos x + c{e^{ - x}}} \over {x\sin x}} = 2x→0lim​xsinxaex−bcosx+ce−x​=2, then a + b + c is equal to ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 4

  1. We need to evaluate
lim⁡x→0aex−bcos⁡x+ce−xxsin⁡x=2.\lim_{x\to 0} \frac{ae^x-b\cos x+ce^{-x}}{x\sin x}=2.x→0lim​xsinxaex−bcosx+ce−x​=2.

Since the denominator tends to 000 as x→0x\to 0x→0, the numerator must also vanish to suitable order.

  1. Use Taylor expansions near x=0x=0x=0:
ex=1+x+x22+⋯ ,e^x=1+x+\frac{x^2}{2}+\cdots,ex=1+x+2x2​+⋯, e−x=1−x+x22+⋯ ,e^{-x}=1-x+\frac{x^2}{2}+\cdots,e−x=1−x+2x2​+⋯, cos⁡x=1−x22+⋯ ,\cos x=1-\frac{x^2}{2}+\cdots,cosx=1−2x2​+⋯, sin⁡x=x+⋯\sin x=x+\cdotssinx=x+⋯

So

xsin⁡x=x(x+⋯ )=x2+⋯x\sin x = x\left(x+\cdots\right)=x^2+\cdotsxsinx=x(x+⋯)=x2+⋯
  1. Expand the numerator:
aex−bcos⁡x+ce−xae^x-b\cos x+ce^{-x}aex−bcosx+ce−x =a(1+x+x22+⋯ )−b(1−x22+⋯ )+c(1−x+x22+⋯ ).= a\left(1+x+\frac{x^2}{2}+\cdots\right)-b\left(1-\frac{x^2}{2}+\cdots\right)+c\left(1-x+\frac{x^2}{2}+\cdots\right).=a(1+x+2x2​+⋯)−b(1−2x2​+⋯)+c(1−x+2x2​+⋯).

Collect terms:

=(a−b+c)+(a−c)x+(a2+b2+c2)x2+⋯= (a-b+c) + (a-c)x + \left(\frac a2+\frac b2+\frac c2\right)x^2+\cdots=(a−b+c)+(a−c)x+(2a​+2b​+2c​)x2+⋯
  1. For the limit to be finite, the constant and linear terms must vanish:
  • Constant term:
a−b+c=0a-b+c=0a−b+c=0
  • Coefficient of xxx:
a−c=0  ⟹  a=ca-c=0 \implies a=ca−c=0⟹a=c

Substitute a=ca=ca=c into the first equation:

a−b+a=0  ⟹  2a−b=0  ⟹  b=2a.a-b+a=0 \implies 2a-b=0 \implies b=2a.a−b+a=0⟹2a−b=0⟹b=2a.
  1. Then the coefficient of x2x^2x2 in the numerator is
a+b+c2.\frac{a+b+c}{2}.2a+b+c​.

Since

xsin⁡x∼x2,x\sin x \sim x^2,xsinx∼x2,

we get

lim⁡x→0aex−bcos⁡x+ce−xxsin⁡x=a+b+c2.\lim_{x\to 0} \frac{ae^x-b\cos x+ce^{-x}}{x\sin x} = \frac{a+b+c}{2}.x→0lim​xsinxaex−bcosx+ce−x​=2a+b+c​.

Given this limit equals 222,

a+b+c2=2.\frac{a+b+c}{2}=2.2a+b+c​=2.

Hence,

a+b+c=4.a+b+c=4.a+b+c=4.
  1. Therefore, the required integer is
4.\boxed{4}.4​.

Comparison with stored answer: the stored correct answer is 444, which matches our result.

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