arthham

JEE Advanced 2017 · Applications of Integrals

If the line sx =α= \alpha divides the area of region R={(x,y)R = \{(x,y) in R2:x3≤y≤x,0≤x≤1}R^{2} : x^{3} \le y \le x, 0 \le x \le 1\} into two equal parts, then (1)0<α≤12(2)12<α<1(3)2α4−4α2+1=0(4)α4+4α2−1=0(1) 0 < \alpha \le \frac{1}{2} (2) \frac{1}{2} < \alpha < 1 (3) 2 \alpha^{4} - 4 \alpha^{2} + 1 = 0 (4) \alpha^{4} + 4 \alpha^{2} - 1 = 0

Answers: B. 12<α<1\frac{1}{2} < \alpha < 1 · C. 2α4−4α2+1=02\alpha^{4} - 4\alpha^{2} + 1 = 0

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Written solution

R={(x,y):x3≤y≤x, 0≤x≤1}R = \{(x,y) : x^3 \le y \le x,\ 0 \le x \le 1\}

vertical line sx=α splits the area in half\text{vertical line } sx = \alpha \text{ splits the area in half}

To find: the correct statement about α\text{To find: the correct statement about } \alpha

x≥x3 on [0,1]: upper y=x, lower y=x3x \ge x^3 \text{ on } [0,1]: \ \text{upper } y=x, \ \text{lower } y=x^3

A=∫01(x−x3) dx=12−14=14A = \int_0^1 (x - x^3)\,dx = \dfrac12 - \dfrac14 = \dfrac14

left part=∫0α(x−x3) dx=α22−α44=18\text{left part} = \int_0^{\alpha}(x - x^3)\,dx = \dfrac{\alpha^2}{2} - \dfrac{\alpha^4}{4} = \dfrac18

4α2−2α4=1 ⇒ 2α4−4α2+1=0 ⇒(3) is true4\alpha^2 - 2\alpha^4 = 1 \ \Rightarrow \ 2\alpha^4 - 4\alpha^2 + 1 = 0 \ \Rightarrow (3)\ \text{is true}

2u2−4u+1=0⇒u=α2=1±222u^2 - 4u + 1 = 0 \Rightarrow u = \alpha^2 = 1 \pm \dfrac{\sqrt2}{2}

α∈(0,1)⇒α2=1−22⇒α≈0.541∈(12,1)\alpha \in (0,1) \Rightarrow \alpha^2 = 1 - \dfrac{\sqrt2}{2} \Rightarrow \alpha \approx 0.541 \in \left(\dfrac12, 1\right)

Answer: statements (2) and (3)\boxed{\text{Answer: statements (2) and (3)}}

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