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JEE Advanced 2022 · Application of Derivatives

Let α=∑k=1∞sin⁡2k(π6)\alpha=\sum_{k=1}^{\infty} \sin ^{2 k}\left(\frac{\pi}{6}\right). Let g:[0,1]→Rg:[0,1] \rightarrow \mathbb{R} be the function defined by g(x)=2αx+2α(1−x)g(x)=2^{\alpha x}+2^{\alpha(1-x)}. Then, which of the following statements is/are TRUE ?

Answer: 11

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Let α=∑ₖ₌₁^∞ sin ^(2 k)(π/6). Let g:[0,1] → R be the function defined by g(x)=2^(α x)+2^(α(1-x)). Then, which of the following statements is/are TRUE ? α=∑k=1∞sin⁡2k ⁣(π6)\alpha = \sum_{k=1}^{\infty} \sin^{2k}\!\left(\dfrac{\pi}{6}\right) g(x)=2αx+2α(1−x),x∈[0,1]g(x) = 2^{\alpha x} + 2^{\alpha(1-x)}, \qquad x \in [0,1] Check: the min and max of g on [0,1],\text{Check: the min and max of } g \text{ on } [0,1], and whether each is attained at more than one point\text{and whether each is attained at more than one point} sin⁡π6=12\sin\dfrac{\pi}{6} = \dfrac{1}{2} α=∑k=1∞(14)k=1/41−1/4=13\alpha = \sum_{k=1}^{\infty}\left(\dfrac{1}{4}\right)^{k} = \dfrac{1/4}{1 - 1/4} = \dfrac{1}{3} g(x)=2x/3+2(1−x)/3,x∈[0,1]g(x) = 2^{x/3} + 2^{(1-x)/3}, \qquad x \in [0,1] 2x/3⋅2(1−x)/3=21/3(constant)2^{x/3}\cdot 2^{(1-x)/3} = 2^{1/3} \quad (\text{constant}) g(x) ≥ 221/3=27/6g(x) \ \ge \ 2\sqrt{2^{1/3}} = 2^{7/6} x/3=(1−x)/3 ⇒ x=12x/3 = (1-x)/3 \ \Rightarrow \ x = \dfrac{1}{2} gmin⁡=27/6 at the single point x=12g_{\min} = 2^{7/6} \text{ at the single point } x = \dfrac{1}{2} u=2x/3∈[1, 21/3],g=u+21/3uu = 2^{x/3} \in [1,\ 2^{1/3}], \qquad g = u + \dfrac{2^{1/3}}{u} g is convex in u⇒the max is at an endpointg \text{ is convex in } u \Rightarrow \text{the max is at an endpoint} g(0)=1+21/3=g(1)g(0) = 1 + 2^{1/3} = g(1) gmax⁡=1+21/3 at x=0 and x=1g_{\max} = 1 + 2^{1/3} \text{ at } x=0 \text{ and } x=1 (1) ✓(2) ✓(3) ✓(4) ×(1)\ \checkmark \quad (2)\ \checkmark \quad (3)\ \checkmark \quad (4)\ \times Answer: (1), (2), (3)\boxed{\text{Answer: (1), (2), (3)}}

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