arthham

JEE Advanced 2024 · Integrals

Let the function f:[1,∞)→Rf: [1, \infty) \to \mathbb{R} be defined by f(t)={(−1)n+12,if t=2n−1,n∈N,(2n+1−t)2f(2n−1)+(t−(2n−1))2f(2n+1),if 2n−1<t<2n+1,n∈N.f(t) = \begin{cases} (-1)^{n+1}2, & \text{if } t = 2n-1, n \in \mathbb{N}, \\ \frac{(2n+1-t)}{2} f(2n-1) + \frac{(t-(2n-1))}{2} f(2n+1), & \text{if } 2n-1 < t < 2n+1, n \in \mathbb{N}. \end{cases} \ Define g(x)=∫1xf(t) dt,x∈(1,∞)g(x) = \int_1^x f(t)\,dt, x \in (1, \infty). Let α\alpha denote the number of solutions of the equation g(x)=0g(x) = 0 in the interval (1,8](1, 8] and β=lim⁡x→1+g(x)x−1\beta = \lim_{x \to 1^+} \frac{g(x)}{x-1}. Then the value of α+β\alpha + \beta is equal to ____.

Answer: 55

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Written solution

Let the function f: [1, ∞) → R be defined by f(t) = (-1)ⁿ⁺¹2, if t = 2n-1, n ∈ N, ((2n+1-t))/2 f(2n-1) + ((t-(2n-1)))/2 f(2n+1), if 2n-1 < t < 2n+1, n ∈ N. Define g(x) = ∫₁ˣ f(t) dt, x ∈ (1, ∞). Let α denote the number of solutions of the equation g(x) = 0 in the interval (1, 8] and β = lim_(x → 1⁺) g(x)/(x-1). Then the value of α + β is equal to ____.

f(2n−1)=(−1)n+12f(2n-1)=(-1)^{n+1}2

f is linear on [2n−1,2n+1]f \text{ is linear on } [2n-1,2n+1]

g(x)=∫1xf(t) dtg(x)=\displaystyle\int_1^x f(t)\,dt

α=#{x∈(1,8]:g(x)=0}\alpha = \#\{x\in(1,8]: g(x)=0\}

β=lim⁡x→1+g(x)x−1\beta = \lim_{x\to1^+}\dfrac{g(x)}{x-1}

find α+β\text{find } \alpha+\beta

1. f on each interval (linear)1.\ f \text{ on each interval (linear)}

2. g(x)=∫1xf, find its zeros2.\ g(x)=\int_1^x f,\ \text{find its zeros}

3. β=g′(1)=f(1)3.\ \beta = g'(1) = f(1)

f(t)=4−2t,t∈[1,3]f(t)=4-2t,\quad t\in[1,3]

f(t)=2t−8,t∈[3,5]f(t)=2t-8,\quad t\in[3,5]

f(t+4)=f(t)⇒period 4f(t+4)=f(t) \Rightarrow \text{period } 4

g(x)=∫1x(4−2t) dt=4x−x2−3g(x)=\int_1^x(4-2t)\,dt = 4x-x^2-3

g(2)=1,g(3)=0g(2)=1,\quad g(3)=0

g(x)=∫3x(2t−8) dt=x2−8x+15g(x)=\int_3^x(2t-8)\,dt = x^2-8x+15

g(4)=−1,g(5)=0g(4)=-1,\quad g(5)=0

g(x+4)=g(x)g(x+4)=g(x)

g=0 at x=3,5,7∈(1,8]g=0 \text{ at } x=3,5,7 \in (1,8]

α=3\alpha = 3

β=lim⁡x→1+g(x)x−1=g′(1)=f(1)=2\beta = \lim_{x\to1^+}\dfrac{g(x)}{x-1} = g'(1) = f(1) = 2

α+β=3+2=5\alpha+\beta = 3+2 = 5

5\boxed{5}

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