MCQMediumJEE 2025Limits

JEE Mathematics 2025 Question with Solution

For α,β,γR\alpha, \beta, \gamma \in \mathbb{R}, if limx0x2sinαx+(γ1)ex23sin2xβx=3,\lim_{x \to 0} \frac{x^2 \sin \alpha x + (\gamma - 1)e^{x^2} - 3}{\sin 2x - \beta x} = 3, then β+γα\beta + \gamma - \alpha is equal to:

  • A

    77

  • B

    44

  • C

    66

  • D

    1-1

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: limx0x2sinαx+(γ1)ex2sin2xβx=3\lim_{x \to 0} \frac{x^2 \sin \alpha x + (\gamma - 1)e^{x^2}}{\sin 2x - \beta x} = 3 for α,β,γR\alpha, \beta, \gamma \in \mathbb{R}.

Find: β+γα\beta + \gamma - \alpha.

Use Maclaurin expansions near x=0x=0.

As x0x \to 0, the denominator tends to 00, so for the limit to be finite the numerator must also tend to 00:

limx0(x2sinαx+(γ1)ex2)=γ1\lim_{x \to 0} \left(x^2 \sin \alpha x + (\gamma - 1)e^{x^2}\right)=\gamma-1

Hence,

γ1=0γ=1\gamma - 1 = 0 \Rightarrow \gamma = 1

Now the numerator becomes

N(x)=x2sin(αx)N(x)=x^2\sin(\alpha x)

Using

sin(αx)=αx(αx)33!+\sin(\alpha x)=\alpha x-\frac{(\alpha x)^3}{3!}+\cdots

we get

N(x)=x2(αxα3x36+)=αx3α3x56+N(x)=x^2\left(\alpha x-\frac{\alpha^3 x^3}{6}+\cdots\right)=\alpha x^3-\frac{\alpha^3 x^5}{6}+\cdots

For the denominator,

D(x)=sin(2x)βxD(x)=\sin(2x)-\beta x

and

sin(2x)=2x(2x)33!+=2x43x3+\sin(2x)=2x-\frac{(2x)^3}{3!}+\cdots=2x-\frac{4}{3}x^3+\cdots

So,

D(x)=(2β)x43x3+D(x)=(2-\beta)x-\frac{4}{3}x^3+\cdots

Since the numerator starts with the term of order x3x^3, the coefficient of xx in the denominator must vanish for the limit to be finite and non-zero:

2β=0β=22-\beta=0 \Rightarrow \beta=2

Then

limx0αx3α3x56+43x3+=3\lim_{x \to 0}\frac{\alpha x^3-\frac{\alpha^3 x^5}{6}+\cdots}{-\frac{4}{3}x^3+\cdots}=3

Comparing coefficients of the lowest power term,

α43=3\frac{\alpha}{-\frac{4}{3}}=3

Thus,

α=3(43)=4\alpha=3\left(-\frac{4}{3}\right)=-4

Now,

β+γα=2+1(4)=7\beta+\gamma-\alpha=2+1-(-4)=7

Therefore, the correct option is A.

The solution concludes that α=4\alpha=-4, β=2\beta=2, and γ=1\gamma=1, so the required value is 77.

Coefficient Matching

Given: the limit equals 33.

Find: β+γα\beta+\gamma-\alpha.

First set x=0x=0 in the numerator condition for finiteness:

(γ1)e0=0γ=1(\gamma-1)e^0=0 \Rightarrow \gamma=1

Then the numerator starts as

x2sin(αx)αx3x^2\sin(\alpha x)\sim \alpha x^3

The denominator is

sin2xβx(2β)x43x3\sin 2x-\beta x \sim (2-\beta)x-\frac{4}{3}x^3

For the limit to be finite and non-zero, the xx-term must vanish:

β=2\beta=2

Now compare the leading x3x^3 coefficients:

α43=3α=4\frac{\alpha}{-\frac{4}{3}}=3 \Rightarrow \alpha=-4

Hence,

β+γα=2+1+4=7\beta+\gamma-\alpha=2+1+4=7

Therefore, the correct option is A.

Common mistakes

  • Setting only the denominator to zero and forgetting that the numerator must also approach 00 for a finite limit. This misses the condition γ1=0\gamma-1=0. First enforce finiteness, then compare series terms.

  • Keeping the linear term (2β)x(2-\beta)x in the denominator. Since the numerator begins with x3x^3, a nonzero linear term would make the limit either 00 or undefined, not a finite non-zero constant. Therefore β=2\beta=2 is necessary.

  • Using the wrong coefficient in sin2x\sin 2x. The correct expansion is sin2x=2x(2x)33!+=2x43x3+\sin 2x=2x-\frac{(2x)^3}{3!}+\cdots=2x-\frac{4}{3}x^3+\cdots. An incorrect cubic coefficient gives a wrong value of α\alpha.

Practice more Limits questions

Get unlimited AI-adaptive practice, mastery tracking, and an AI tutor that explains every step — free to start.

Related questions