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Limits Continuity and Differentiability question

2021 · 20 Jul · Shift 2 · Q45
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  5. /2021 · 20 Jul · Shift 2 · Q45

Limits Continuity and Differentiability question

2021 · 20 Jul · Shift 2 · Q45

JEE MainMathematicsLimits Continuity and DifferentiabilityNumerical+4 / −1
If lim⁡x→0αxex−βlog⁡e(1+x)+γx2e−xxsin⁡2x=10,α,β,γ∈R\mathop {\lim }\limits_{x \to 0} {{\alpha x{e^x} - \beta {{\log }_e}(1 + x) + \gamma {x^2}{e^{ - x}}} \over {x{{\sin }^2}x}} = 10,\alpha ,\beta ,\gamma \in Rx→0lim​xsin2xαxex−βloge​(1+x)+γx2e−x​=10,α,β,γ∈R, then the value of α\alphaα+β\betaβ+γ\gammaγ is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 3

  1. We need to evaluate
lim⁡x→0αxex−βln⁡(1+x)+γx2e−xxsin⁡2x=10.\lim_{x\to 0}\frac{\alpha x e^x-\beta \ln(1+x)+\gamma x^2 e^{-x}}{x\sin^2 x}=10.x→0lim​xsin2xαxex−βln(1+x)+γx2e−x​=10.

Since the denominator tends to 000, for the limit to be finite, the numerator must vanish up to sufficiently high order.


  1. Expand each term near x=0x=0x=0.

We use:

ex=1+x+x22+x36+⋯ ,e^x=1+x+\frac{x^2}{2}+\frac{x^3}{6}+\cdots,ex=1+x+2x2​+6x3​+⋯, e−x=1−x+x22−x36+⋯ ,e^{-x}=1-x+\frac{x^2}{2}-\frac{x^3}{6}+\cdots,e−x=1−x+2x2​−6x3​+⋯, ln⁡(1+x)=x−x22+x33−x44+⋯ ,\ln(1+x)=x-\frac{x^2}{2}+\frac{x^3}{3}-\frac{x^4}{4}+\cdots,ln(1+x)=x−2x2​+3x3​−4x4​+⋯, sin⁡x=x−x36+⋯  ⟹  sin⁡2x=x2−x43+⋯\sin x = x-\frac{x^3}{6}+\cdots \implies \sin^2 x = x^2-\frac{x^4}{3}+\cdotssinx=x−6x3​+⋯⟹sin2x=x2−3x4​+⋯

Hence,

xsin⁡2x=x3+O(x5).x\sin^2 x = x^3+O(x^5).xsin2x=x3+O(x5).

Now expand the numerator:

αxex=αx(1+x+x22+x36+⋯ )=αx+αx2+α2x3+α6x4+⋯\alpha x e^x=\alpha x\left(1+x+\frac{x^2}{2}+\frac{x^3}{6}+\cdots\right) =\alpha x+\alpha x^2+\frac{\alpha}{2}x^3+\frac{\alpha}{6}x^4+\cdotsαxex=αx(1+x+2x2​+6x3​+⋯)=αx+αx2+2α​x3+6α​x4+⋯ −βln⁡(1+x)=−β(x−x22+x33−x44+⋯ )=−βx+β2x2−β3x3+β4x4+⋯-\beta\ln(1+x)=-\beta\left(x-\frac{x^2}{2}+\frac{x^3}{3}-\frac{x^4}{4}+\cdots\right) =-\beta x+\frac{\beta}{2}x^2-\frac{\beta}{3}x^3+\frac{\beta}{4}x^4+\cdots−βln(1+x)=−β(x−2x2​+3x3​−4x4​+⋯)=−βx+2β​x2−3β​x3+4β​x4+⋯ γx2e−x=γx2(1−x+x22−x36+⋯ )=γx2−γx3+γ2x4+⋯\gamma x^2 e^{-x}=\gamma x^2\left(1-x+\frac{x^2}{2}-\frac{x^3}{6}+\cdots\right) =\gamma x^2-\gamma x^3+\frac{\gamma}{2}x^4+\cdotsγx2e−x=γx2(1−x+2x2​−6x3​+⋯)=γx2−γx3+2γ​x4+⋯

Adding,

Numerator=(α−β)x+(α+β2+γ)x2+(α2−β3−γ)x3+⋯\text{Numerator}=(\alpha-\beta)x+\left(\alpha+\frac{\beta}{2}+\gamma\right)x^2 +\left(\frac{\alpha}{2}-\frac{\beta}{3}-\gamma\right)x^3+\cdotsNumerator=(α−β)x+(α+2β​+γ)x2+(2α​−3β​−γ)x3+⋯
  1. Since denominator is of order x3x^3x3, for the limit to be finite we need coefficients of xxx and x2x^2x2 in the numerator to be zero.

So,

α−β=0⇒α=β\alpha-\beta=0 \quad\Rightarrow\quad \alpha=\betaα−β=0⇒α=β

and

α+β2+γ=0.\alpha+\frac{\beta}{2}+\gamma=0.α+2β​+γ=0.

Using α=β\alpha=\betaα=β,

α+α2+γ=0⇒γ=−3α2.\alpha+\frac{\alpha}{2}+\gamma=0 \Rightarrow \gamma=-\frac{3\alpha}{2}.α+2α​+γ=0⇒γ=−23α​.
  1. Now use the value of the limit.

Because

xsin⁡2x∼x3,x\sin^2x \sim x^3,xsin2x∼x3,

the limit equals the coefficient of x3x^3x3 in the numerator:

α2−β3−γ=10.\frac{\alpha}{2}-\frac{\beta}{3}-\gamma=10.2α​−3β​−γ=10.

Since β=α\beta=\alphaβ=α and γ=−3α2\gamma=-\frac{3\alpha}{2}γ=−23α​,

α2−α3+3α2=10.\frac{\alpha}{2}-\frac{\alpha}{3}+\frac{3\alpha}{2}=10.2α​−3α​+23α​=10.

Combine terms:

α2+3α2=2α,\frac{\alpha}{2}+\frac{3\alpha}{2}=2\alpha,2α​+23α​=2α,

so

2α−α3=102\alpha-\frac{\alpha}{3}=102α−3α​=10 6α−α3=10\frac{6\alpha-\alpha}{3}=1036α−α​=10 5α3=10⇒α=6.\frac{5\alpha}{3}=10 \Rightarrow \alpha=6.35α​=10⇒α=6.

Hence,

β=6,\beta=6,β=6, γ=−3⋅62=−9.\gamma=-\frac{3\cdot 6}{2}=-9.γ=−23⋅6​=−9.
  1. Therefore,
α+β+γ=6+6−9=3.\alpha+\beta+\gamma=6+6-9=3.α+β+γ=6+6−9=3.

So the required integer is

3.\boxed{3}.3​.
  1. Comparison with stored answer:

Stored correct answer = 333. Our derived answer also = 333. Hence, they agree.

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