JEE Challenger
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Limit of Logarithmic Trigonometric Ratio and Derivative Value

Let
g(x)=(x−1)nlog⁡cos⁡m(x−1);0<x<2,m and n are integers,m≠0,n>0,g(x) = \frac{(x-1)^n}{\log \cos^m (x-1)}; \quad 0 < x < 2, \quad m \text{ and } n \text{ are integers}, \quad m \neq 0, \quad n > 0,
and let pp be the left hand derivative of ∣x−1∣|x-1| at x=1x = 1.

If lim⁡x→1+g(x)=p\lim_{x \to 1^+} g(x) = p, then

Options

A

n=1, m=1n = 1, \, m = 1

B

n=1, m=−1n = 1, \, m = -1

C

n=2, m=2n = 2, \, m = 2

Correct
D

n>2, m=nn > 2, \, m = n

Step-by-Step Solution

To find the values of nn and mm, we first determine the value of pp, which is defined as the left-hand derivative of ∣x−1∣|x-1| at x=1x = 1.

By the definition of the left-hand derivative:
p=lim⁡h→0−∣(1+h)−1∣−∣1−1∣h=lim⁡h→0−∣h∣hp = \lim_{h \to 0^-} \frac{|(1 + h) - 1| - |1 - 1|}{h} = \lim_{h \to 0^-} \frac{|h|}{h}

Since h<0h < 0, we have ∣h∣=−h|h| = -h. Therefore:
p=lim⁡h→0−−hh=−1p = \lim_{h \to 0^-} \frac{-h}{h} = -1

Now, we evaluate the given limit:
lim⁡x→1+g(x)=lim⁡x→1+(x−1)nlog⁡cos⁡m(x−1)\lim_{x \to 1^+} g(x) = \lim_{x \to 1^+} \frac{(x-1)^n}{\log \cos^m (x-1)}

Let t=x−1t = x - 1. As x→1+x \to 1^+, t→0+t \to 0^+. Since cos⁡t>0\cos t > 0 for t∈(0,1)t \in (0, 1), we can rewrite the logarithmic term using the power property of logarithms:
log⁡cos⁡m(t)=mlog⁡(cos⁡t)\log \cos^m(t) = m \log(\cos t)

Thus, the limit becomes:
lim⁡t→0+tnmlog⁡(cos⁡t)\lim_{t \to 0^+} \frac{t^n}{m \log(\cos t)}

Using the Taylor series expansion of cos⁡t\cos t near t=0t = 0:
cos⁡t=1−t22+O(t4)\cos t = 1 - \frac{t^2}{2} + O(t^4)

Applying the expansion of log⁡(1+u)\log(1 + u) for u=−t22+O(t4)u = -\frac{t^2}{2} + O(t^4):
log⁡(cos⁡t)=log⁡(1−t22+O(t4))=−t22+O(t4)\log(\cos t) = \log\left(1 - \frac{t^2}{2} + O(t^4)\right) = -\frac{t^2}{2} + O(t^4)

Substituting this back into the limit:
lim⁡t→0+tnm(−t22+O(t4))=lim⁡t→0+−2m⋅tnt2(1+O(t2))=lim⁡t→0+(−2m)tn−2\lim_{t \to 0^+} \frac{t^n}{m \left(-\frac{t^2}{2} + O(t^4)\right)} = \lim_{t \to 0^+} -\frac{2}{m} \cdot \frac{t^n}{t^2 (1 + O(t^2))} = \lim_{t \to 0^+} \left(-\frac{2}{m}\right) t^{n-2}

For the limit to exist and equal the non-zero finite value p=−1p = -1:

  1. The exponent of tt must be zero:
    n−2=0  ⟹  n=2n - 2 = 0 \implies n = 2

  2. The coefficient must equal pp:
    −2m=−1  ⟹  m=2-\frac{2}{m} = -1 \implies m = 2

Thus, we have n=2n = 2 and m=2m = 2.

Therefore, the correct option is C.