Sign in
Please select an account to continue using cracku.in
↓ →
Join Our JEE Preparation Group
Prep with like-minded aspirants; Get access to free daily tests and study material.
If $$\lim_{x \to 2} \frac{\sin(x^3 - 5x^2 + ax + b)}{(\sqrt{x-1} - 1) \log_e(x-1)} = m$$, then $$a + b + m$$ is equal to :
Let the denominator be $$D(x) = (\sqrt{x-1} - 1) \ln(x-1)$$.
As $$x \to 2$$:
Since the denominator approaches $$0$$, the numerator must also approach $$0$$ for the limit to be finite.
The numerator is $$N(x) = \sin(x^3 - 5x^2 + ax + b)$$.
For $$\lim_{x \to 2} N(x) = 0$$, the argument of the sine function must be $$0$$:
$$2^3 - 5(2^2) + a(2) + b = 0$$ $$8 - 20 + 2a + b = 0 \implies 2a + b = 12 \quad \text{--- (Eq. 1)}$$
Furthermore, since the denominator has two factors that both go to zero, the numerator's argument must contain $$(x-2)^2$$ as a factor to cancel out the "intensity" of the zero in the denominator. Let $$f(x) = x^3 - 5x^2 + ax + b$$. For $$(x-2)^2$$ to be a factor, $$f(2) = 0$$ and $$f'(2) = 0$$.
$$f'(x) = 3x^2 - 10x + a$$ $$f'(2) = 3(4) - 10(2) + a = 0 \implies 12 - 20 + a = 0 \implies \mathbf{a = 8}$$
Substitute $$a = 8$$ into (Eq. 1):
$$2(8) + b = 12 \implies 16 + b = 12 \implies \mathbf{b = -4}$$
Now we evaluate the limit with $$a=8, b=-4$$:
$$\lim_{x \to 2} \frac{\sin(x^3 - 5x^2 + 8x - 4)}{(\sqrt{x-1}-1) \ln(1+(x-2))}$$
Since $$\sin \theta \approx \theta$$ and $$\ln(1+\theta) \approx \theta$$ as $$\theta \to 0$$:
$$m = \lim_{x \to 2} \frac{x^3 - 5x^2 + 8x - 4}{(\sqrt{x-1}-1)(x-2)}$$
Factor the numerator: $$x^3 - 5x^2 + 8x - 4 = (x-2)^2(x-1)$$.
Rationalize the denominator part: $$(\sqrt{x-1}-1) = \frac{(x-1)-1}{\sqrt{x-1}+1} = \frac{x-2}{\sqrt{x-1}+1}$$.
Substitute these:
$$m = \lim_{x \to 2} \frac{(x-2)^2(x-1)}{\frac{x-2}{\sqrt{x-1}+1} \cdot (x-2)} = \lim_{x \to 2} (x-1)(\sqrt{x-1}+1)$$
$$m = (2-1)(\sqrt{2-1}+1) = 1(1+1) = \mathbf{2}$$
The question asks for $a + b + m$:
$$a + b + m = 8 + (-4) + 2 = 6$$
Correct Option: B (6)
Create a FREE account and get:
Predict your JEE Main percentile, rank & performance in seconds
Educational materials for JEE preparation
Ask our AI anything
AI can make mistakes. Please verify important information.
AI can make mistakes. Please verify important information.