NVAMediumJEE 2024Differentiability

JEE Mathematics 2024 Question with Solution

If the function f(x)={1x,x2ax2+2b,x<2f(x) = \left\{ \frac{1}{|x|} , |x| \geq 2 \\ ax^2 + 2b , |x| < 2 \right. is differentiable on RR, then 48(a+b)48(a + b) is equal to:

Answer

Correct answer:15

Step-by-step solution

Standard Method

Given: The piecewise function is

f(x)={1x,x2ax2+2b,2<x<21x,x2f(x) = \begin{cases} -\frac{1}{x}, & x \leq -2 \\ ax^2 + 2b, & -2 < x < 2 \\ \frac{1}{x}, & x \geq 2 \end{cases}

and it is differentiable on R\mathbb{R}.

Find: The value of 48(a+b)48(a+b).

For differentiability, the function must be both continuous and differentiable at the boundary points x=2x=2 and x=2x=-2.

Using continuity at x=2x=2:

4a+2b=124a + 2b = \frac{1}{2}

Using continuity at x=2x=-2 gives the same equation:

4a+2b=124a + 2b = \frac{1}{2}

Now differentiate each part:

f(x)={1x2,x<22ax,2<x<21x2,x>2f'(x) = \begin{cases} \frac{1}{x^2}, & x < -2 \\ 2ax, & -2 < x < 2 \\ -\frac{1}{x^2}, & x > 2 \end{cases}

Using differentiability at x=2x=2:

2a(2)=1222a(2) = -\frac{1}{2^2} 4a=144a = -\frac{1}{4} a=116a = -\frac{1}{16}

Using differentiability at x=2x=-2 also gives the same value:

1(2)2=2a(2)\frac{1}{(-2)^2} = 2a(-2) 14=4a\frac{1}{4} = -4a a=116a = -\frac{1}{16}

Substitute a=116a=-\frac{1}{16} into the continuity equation:

4(116)+2b=124\left(-\frac{1}{16}\right) + 2b = \frac{1}{2} 14+2b=12-\frac{1}{4} + 2b = \frac{1}{2} 2b=342b = \frac{3}{4} b=38b = \frac{3}{8}

Now compute:

a+b=116+38=516a+b = -\frac{1}{16} + \frac{3}{8} = \frac{5}{16} 48(a+b)=48×516=1548(a+b) = 48 \times \frac{5}{16} = 15

Therefore, the value of 48(a+b)48(a+b) is 1515.

Continuity and Differentiability at Both Junctions

Given: The definition changes at x=2|x|=2, so the critical points are x=2x=2 and x=2x=-2.

Find: Determine aa and bb, then evaluate 48(a+b)48(a+b).

At x=2x=2, continuity requires

limx2(ax2+2b)=limx2+1x\lim_{x \to 2^-}(ax^2+2b)=\lim_{x \to 2^+}\frac{1}{x} 4a+2b=124a+2b=\frac{1}{2}

At x=2x=-2, continuity requires

limx2(1x)=limx2+(ax2+2b)\lim_{x \to -2^-}\left(-\frac{1}{x}\right)=\lim_{x \to -2^+}(ax^2+2b) 12=4a+2b\frac{1}{2}=4a+2b

So continuity gives the single equation

4a+2b=124a+2b=\frac{1}{2}

For differentiability at x=2x=2:

limx2(2ax)=limx2+(1x2)\lim_{x \to 2^-}(2ax)=\lim_{x \to 2^+}\left(-\frac{1}{x^2}\right) 4a=144a=-\frac{1}{4} a=116a=-\frac{1}{16}

For differentiability at x=2x=-2:

limx21x2=limx2+(2ax)\lim_{x \to -2^-}\frac{1}{x^2}=\lim_{x \to -2^+}(2ax) 14=4a\frac{1}{4}=-4a a=116a=-\frac{1}{16}

This confirms the same value of aa.

Now solve for bb:

4(116)+2b=124\left(-\frac{1}{16}\right)+2b=\frac{1}{2} 14+2b=12-\frac{1}{4}+2b=\frac{1}{2} 2b=342b=\frac{3}{4} b=38b=\frac{3}{8}

Finally,

a+b=116+38=516a+b=-\frac{1}{16}+\frac{3}{8}=\frac{5}{16} 48(a+b)=48516=1548(a+b)=48\cdot\frac{5}{16}=15

Hence, the required numerical value is 1515.

Common mistakes

  • Checking only continuity and not differentiability at x=±2x=\pm 2 is incorrect, because the question explicitly says the function is differentiable on R\mathbb{R}. After matching function values, also equate the derivatives from both sides.

  • Differentiating 1x\frac{1}{|x|} incorrectly near negative values is a common error. For x2x\le -2, x=x|x|=-x, so 1x=1x\frac{1}{|x|}=-\frac{1}{x}, whose derivative is 1x2\frac{1}{x^2}.

  • Using the wrong derivative for the positive branch at x>2x>2 leads to sign mistakes. Since f(x)=1xf(x)=\frac{1}{x} there, the derivative is 1x2-\frac{1}{x^2}, not 1x2\frac{1}{x^2}.

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