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JEE Advanced 2016 · Continuity and Differentiability

Let f:R→Rf:\mathbb{R}\to\mathbb{R}, g:R→Rg:\mathbb{R}\to\mathbb{R} and h:R→Rh:\mathbb{R}\to\mathbb{R} be differentiable functions such that f(x)=x3+3x+2f(x)=x^{3}+3x+2, g(f(x))=xg(f(x))=x and h(g(g(x)))=xh(g(g(x)))=x for all x∈Rx\in\mathbb{R}. Then

Answers: B. h′(1)=666h'(1)=666 · C. h(0)=16h(0)=16

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f(x)=x3+3x+2f(x) = x^3 + 3x + 2 g(f(x))=x,h(g(g(x)))=xg(f(x)) = x, \quad h(g(g(x))) = x Which of the four statements is/are correct?\text{Which of the four statements is/are correct?} f′(x)=3x2+3>0⇒f is invertible, g=f−1f'(x) = 3x^2 + 3 > 0 \Rightarrow f \text{ is invertible},\ g = f^{-1} g(2)=0 since f(0)=2g(2) = 0 \text{ since } f(0) = 2 g′(2)=1f′(0)=13≠115⇒(1) is falseg'(2) = \dfrac{1}{f'(0)} = \dfrac{1}{3} \neq \dfrac{1}{15} \Rightarrow (1) \text{ is false} h∘g∘g=id⇒h=(g∘g)−1=f∘fh \circ g \circ g = \mathrm{id} \Rightarrow h = (g \circ g)^{-1} = f \circ f h′(x)=f′(f(x)) f′(x)h'(x) = f'(f(x))\,f'(x) h′(1)=f′(6)⋅f′(1)=111⋅6=666⇒(2) is trueh'(1) = f'(6)\cdot f'(1) = 111 \cdot 6 = 666 \Rightarrow (2) \text{ is true} h(0)=f(f(0))=f(2)=16⇒(3) is trueh(0) = f(f(0)) = f(2) = 16 \Rightarrow (3) \text{ is true} h(g(3))=f(f(g(3)))=f(3)=38≠36⇒(4) is falseh(g(3)) = f(f(g(3))) = f(3) = 38 \neq 36 \Rightarrow (4) \text{ is false} Correct options: (2) and (3)\boxed{\text{Correct options: } (2) \text{ and } (3)}

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