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Limits Continuity and Differentiability question

2025 · 23 Jan · Shift 1 · Q34
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  5. /2025 · 23 Jan · Shift 1 · Q34

Limits Continuity and Differentiability question

2025 · 23 Jan · Shift 1 · Q34

JEE MainMathematicsLimits Continuity and DifferentiabilityMCQ+4 / −1
If the function f(x)={2x{sin⁡(k1+1)x+sin⁡(k2−1)x},x0f(x)=\left\{\begin{array}{l} \frac{2}{x}\left\{\sin \left(k_1+1\right) x+\sin \left(k_2-1\right) x\right\}, \quad x0 \end{array}\right.f(x)={x2​{sin(k1​+1)x+sin(k2​−1)x},x0​ is continuous at x=0x=0x=0, then k12+k22k_1^2+k_2^2k12​+k22​ is equal to :
  1. A
    5
  2. B
    10
  3. C
    20
  4. D
    8
View written solutionFree

Correct answer: QUESTION APPEARS INCOMPLETE/MISPRINTED., FROM CONTINUITY AT $X=0$: $2(K_1+K_2)=8\RIGHTARROW K_1+K_2=4$., THUS $K_1^2+K_2^2$ IS NOT UNIQUELY DETERMINED.

  1. Interpret the function

    The given piecewise function is intended as

    f(x)={2x[sin⁡((k1+1)x)+sin⁡((k2−1)x)],x≠0,8,x=0.f(x)= \begin{cases} \dfrac{2}{x}\left[\sin\big((k_1+1)x\big)+\sin\big((k_2-1)x\big)\right], & x\ne 0,\\[6pt] 8, & x=0. \end{cases}f(x)=⎩⎨⎧​x2​[sin((k1​+1)x)+sin((k2​−1)x)],8,​x=0,x=0.​

    We are told that f(x)f(x)f(x) is continuous at x=0x=0x=0.

  2. Condition for continuity at x=0x=0x=0

    For continuity at x=0x=0x=0, we need

    lim⁡x→0f(x)=f(0)=8.\lim_{x\to 0} f(x)=f(0)=8.x→0lim​f(x)=f(0)=8.

    So compute the limit:

    lim⁡x→02x[sin⁡((k1+1)x)+sin⁡((k2−1)x)].\lim_{x\to 0} \frac{2}{x}\left[\sin\big((k_1+1)x\big)+\sin\big((k_2-1)x\big)\right].x→0lim​x2​[sin((k1​+1)x)+sin((k2​−1)x)].
  3. Use the standard limit

    Recall:

    lim⁡x→0sin⁡(ax)x=a.\lim_{x\to 0}\frac{\sin(ax)}{x}=a.x→0lim​xsin(ax)​=a.

    Therefore,

    lim⁡x→02xsin⁡((k1+1)x)=2(k1+1),\lim_{x\to 0} \frac{2}{x}\sin\big((k_1+1)x\big)=2(k_1+1),x→0lim​x2​sin((k1​+1)x)=2(k1​+1),

    and

    lim⁡x→02xsin⁡((k2−1)x)=2(k2−1).\lim_{x\to 0} \frac{2}{x}\sin\big((k_2-1)x\big)=2(k_2-1).x→0lim​x2​sin((k2​−1)x)=2(k2​−1).

    Hence,

    lim⁡x→0f(x)=2(k1+1)+2(k2−1)=2(k1+k2).\lim_{x\to 0} f(x)=2(k_1+1)+2(k_2-1)=2(k_1+k_2).x→0lim​f(x)=2(k1​+1)+2(k2​−1)=2(k1​+k2​).
  4. Apply continuity

    Since f(0)=8f(0)=8f(0)=8,

    2(k1+k2)=82(k_1+k_2)=82(k1​+k2​)=8 k1+k2=4.k_1+k_2=4.k1​+k2​=4.
  5. Find k12+k22k_1^2+k_2^2k12​+k22​ from the options

    The relation obtained is only

    k1+k2=4.k_1+k_2=4.k1​+k2​=4.

    Now check the options for possible values of k12+k22k_1^2+k_2^2k12​+k22​.

    Using

    k12+k22=(k1+k2)2−2k1k2=16−2k1k2,k_1^2+k_2^2=(k_1+k_2)^2-2k_1k_2=16-2k_1k_2,k12​+k22​=(k1​+k2​)2−2k1​k2​=16−2k1​k2​,

    this is not uniquely determined unless there is some further condition on k1,k2k_1,k_2k1​,k2​.

    However, among standard exam interpretations, k1,k2k_1,k_2k1​,k2​ are often taken as positive integers. Then with

    k1+k2=4,k_1+k_2=4,k1​+k2​=4,

    possible pairs are:

    • (1,3)(1,3)(1,3) giving 12+32=101^2+3^2=1012+32=10
    • (2,2)(2,2)(2,2) giving 22+22=82^2+2^2=822+22=8
    • (3,1)(3,1)(3,1) giving 101010

    So even under integer assumption, the value is still not unique.

  6. Compare with the stored answer

    The stored answer is B: 10.

    But from the continuity condition alone, we only get

    k1+k2=4,k_1+k_2=4,k1​+k2​=4,

    and hence k12+k22k_1^2+k_2^2k12​+k22​ cannot be uniquely determined. So the question as written appears incomplete or misprinted.

  7. Conclusion

    The mathematically correct conclusion is:

    • Continuity implies k1+k2=4k_1+k_2=4k1​+k2​=4.
    • Therefore k12+k22k_1^2+k_2^2k12​+k22​ is not uniquely determined from the given information.
    • So the stored answer 101010 cannot be justified from the question as stated.
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