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JEE Advanced 2024 · Continuity and Differentiability

Let f:R→Rf:\mathbb{R}\to\mathbb{R} and g:R→Rg:\mathbb{R}\to\mathbb{R} be functions defined by f(x)={x∣x∣sin⁡(1x),x≠00,x=0f(x)=\begin{cases} x|x|\sin\left(\frac{1}{x}\right), & x\neq 0 \\ 0, & x=0 \end{cases} and g(x)={1−2x,0≤x≤12,0,otherwise.g(x)=\begin{cases} 1-2x, & 0\le x\le \frac{1}{2}, \\ 0, & \text{otherwise}. \end{cases} Let a,b,c,d∈Ra,b,c,d\in\mathbb{R}. Define the function h:R→Rh:\mathbb{R}\to\mathbb{R} by h(x)=af(x)+b(g(x)+g(12−x))+c(x−g(x))+d g(x)h(x)=a f(x)+b\left(g(x)+g\left(\frac{1}{2}-x\right)\right)+c(x-g(x))+d\,g(x), x∈Rx\in\mathbb{R}. Match each entry in List-I to the correct entry in List-II. | List-I | List-II | | --- | --- | | (P) If a=0a=0, b=1b=1, c=0c=0, and d=0d=0, then | (1) hh is one-one. | | (Q) If a=1a=1, b=0b=0, c=0c=0, and d=0d=0, then | (2) hh is onto. | | (R) If a=0a=0, b=0b=0, c=1c=1, and d=0d=0, then | (3) hh is differentiable on R\mathbb{R} | | (S) If a=0a=0, b=0b=0, c=0c=0, and d=1d=1 then | (4) the range of hh is [0,1][0,1] | | | (5) the range of hh is {0,1}\{0,1\}. |

Answer: C. (P)→\to(5) (Q)→\to(3) (R)→\to(2) (S)→\to(4)

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Written solution

f(x)= x|x|sin(1/x), x≠ 0 0, x=0 g(x)= 1-2x, 0≤ x≤ 1/2 0, otherwise

h(x)=af(x)+b(g(x)+g(12−x))+c(x−g(x))+d g(x)h(x)=a f(x)+b\left(g(x)+g\left(\frac{1}{2}-x\right)\right)+c(x-g(x))+d\,g(x)

Match P, Q, R, S to (1)–(5)\text{Match P, Q, R, S to (1)--(5)}

To find: the correct option\text{To find: the correct option}

0≤x≤12: g(x)=1−2x,g(12−x)=1−2(12−x)=2x0\le x\le \tfrac12:\ g(x)=1-2x,\quad g\left(\tfrac12-x\right)=1-2\left(\tfrac12-x\right)=2x ⇒g(x)+g(12−x)={1,0≤x≤120,otherwise\Rightarrow g(x)+g\left(\tfrac12-x\right) = \begin{cases} 1, & 0\le x\le \tfrac12 \\ 0, & \text{otherwise}\end{cases} P: h=SP:\ h=S Q: h=fQ:\ h=f R: h=x−g(x)R:\ h=x-g(x) S: h=gS:\ h=g P: h(x)={1,0≤x≤120,otherwise⇒range {0,1}⇒(5)P:\ h(x)=\begin{cases}1,& 0\le x\le\tfrac12\\ 0,&\text{otherwise}\end{cases} \Rightarrow \text{range } \{0,1\} \Rightarrow (5) ∣f(x)∣=x2∣sin⁡1x∣≤x2⇒lim⁡x→0f(x)=0=f(0)|f(x)| = x^2\left|\sin\tfrac1x\right| \le x^2 \Rightarrow \lim_{x\to0}f(x)=0=f(0) f′(0)=lim⁡x→0f(x)−0x=lim⁡x→0∣x∣sin⁡1x=0f'(0) = \lim_{x\to0}\dfrac{f(x)-0}{x} = \lim_{x\to0}|x|\sin\tfrac1x = 0 ⇒h=f differentiable on R⇒(3)\Rightarrow h=f \text{ differentiable on } \mathbb{R} \Rightarrow (3) R: h(x)=x−g(x)={3x−1,0≤x≤12x,otherwiseR:\ h(x)=x-g(x)=\begin{cases} 3x-1, & 0\le x\le\tfrac12 \\ x, & \text{otherwise}\end{cases} range=(−∞,0)∪[−1,12]∪(12,∞)=R⇒onto⇒(2)\text{range} = (-\infty,0)\cup[-1,\tfrac12]\cup(\tfrac12,\infty)=\mathbb{R} \Rightarrow \text{onto} \Rightarrow (2) S: h=g⇒range=[0,1]⇒(4)S:\ h=g \Rightarrow \text{range} = [0,1] \Rightarrow (4) P→5,Q→3,R→2,S→4P\to5,\quad Q\to3,\quad R\to2,\quad S\to4 option (3)\boxed{\text{option (3)}}

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