arthham

JEE Advanced 2021 · Integrals

Let f:[−π2,π2]→Rf:\left[-\frac{\pi}{2}, \frac{\pi}{2}\right] \to \mathbb{R} be a continuous function such that f(0)=1f(0) = 1 and ∫0π3f(t)dt=0\int_{0}^{\frac{\pi}{3}} f(t)dt = 0. Then which of the following statements is (are) **TRUE**?

Answers: A. The equation f(x)−3cos⁡3x=0f(x) - 3\cos 3x = 0 has at least one solution in (0,π3)\left(0, \frac{\pi}{3}\right) · B. The equation f(x)−3sin⁡3x=−6πf(x) - 3\sin 3x = -\frac{6}{\pi} has at least one solution in (0,π3)\left(0, \frac{\pi}{3}\right) · C. lim⁡x→∞x∫0xf(t)dt1−ex2=−1\lim_{x \to \infty} \frac{x\int_{0}^{x} f(t)dt}{1 - e^{x^{2}}} = -1

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f continuous on [−π2,π2],f(0)=1f \text{ continuous on } \left[-\tfrac{\pi}{2}, \tfrac{\pi}{2}\right], \qquad f(0) = 1 ∫0π/3f(t) dt=0\int_{0}^{\pi/3} f(t)\,dt = 0 Existence of roots in (0,π/3); and two limits\text{Existence of roots in } (0,\pi/3); \text{ and two limits} F(x)=∫0xf(t) dt;F(0)=0, F ⁣(π3)=0, F′(x)=f(x)F(x) = \int_{0}^{x} f(t)\,dt; \quad F(0)=0,\ F\!\left(\tfrac{\pi}{3}\right)=0,\ F'(x)=f(x) F′(0)=f(0)=1F'(0) = f(0) = 1 G(x)=F(x)−sin⁡3x;G(0)=0, G ⁣(π3)=0G(x) = F(x) - \sin 3x; \quad G(0) = 0, \ G\!\left(\tfrac{\pi}{3}\right) = 0 ⇒∃ c∈(0,π3): f(c)−3cos⁡3c=0⇒(1) TRUE\Rightarrow \exists\, c \in \left(0,\tfrac{\pi}{3}\right):\ f(c) - 3\cos 3c = 0 \Rightarrow (1) \text{ TRUE} G(x)=F(x)+cos⁡3x+6πx;G(0)=1, G ⁣(π3)=0−1+2=1G(x) = F(x) + \cos 3x + \tfrac{6}{\pi}x; \quad G(0) = 1, \ G\!\left(\tfrac{\pi}{3}\right) = 0 - 1 + 2 = 1 ⇒∃ c: f(c)−3sin⁡3c=−6π⇒(2) TRUE\Rightarrow \exists\, c:\ f(c) - 3\sin 3c = -\tfrac{6}{\pi} \Rightarrow (2) \text{ TRUE} domain bounded⇒the limits are x→0\text{domain bounded} \Rightarrow \text{the limits are } x \to 0 F(x)=x+o(x) as x→0F(x) = x + o(x) \text{ as } x \to 0 lim⁡x→0x F(x)1−ex2∼x⋅x−x2=−1⇒(3) TRUE\lim_{x \to 0} \dfrac{x\,F(x)}{1 - e^{x^{2}}} \sim \dfrac{x\cdot x}{-x^{2}} = -1 \Rightarrow (3) \text{ TRUE} lim⁡x→0sin⁡x F(x)x2∼x⋅xx2=1≠−1⇒(4) FALSE\lim_{x \to 0} \dfrac{\sin x\,F(x)}{x^{2}} \sim \dfrac{x\cdot x}{x^{2}} = 1 \neq -1 \Rightarrow (4) \text{ FALSE} TRUE statements: (1),(2),(3)\boxed{\text{TRUE statements: } (1), (2), (3)} F=∫0xf; Rolle⇒(1),(2); x→0⇒(3) TRUE,(4) FALSEF = \int_0^x f;\ \text{Rolle} \Rightarrow (1),(2);\ x \to 0 \Rightarrow (3) \text{ TRUE}, (4) \text{ FALSE}

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