JEE Challenger
More from Application of Integrals

Area Division by Vertical Line for Cubic Region

Suppose the line x=αx = \alpha divides the area bounded by the region R={(x,y)∈R2:x3≤y≤x,0≤x≤1}R = \{(x, y) \in \mathbb{R}^2 : x^3 \le y \le x, 0 \le x \le 1\} into two equal parts. Then:

Options

A

0<α≤120 < \alpha \le \frac{1}{2}

B

12<α<1\frac{1}{2} < \alpha < 1

Correct
C

2α4−4α2+1=02\alpha^4 - 4\alpha^2 + 1 = 0

Correct
D

α4+4α2−1=0\alpha^4 + 4\alpha^2 - 1 = 0

Step-by-Step Solution

To determine the correct options, we first calculate the total area of the region RR, bounded by y=x3y = x^3 and y=xy = x for 0≤x≤10 \le x \le 1.

Step 1: Calculate the Total Area of Region RR

The total area AA is given by the definite integral: A=∫01(x−x3) dxA = \int_{0}^{1} (x - x^3) \, dx

Evaluating the integral: A=[x22−x44]01=12−14=14A = \left[ \frac{x^2}{2} - \frac{x^4}{4} \right]_{0}^{1} = \frac{1}{2} - \frac{1}{4} = \frac{1}{4}


Step 2: Establish the Equation for α\alpha

The line x=αx = \alpha divides the region RR into two equal parts, so the area from x=0x = 0 to x=αx = \alpha must be equal to half of the total area AA: ∫0α(x−x3) dx=12×14=18\int_{0}^{\alpha} (x - x^3) \, dx = \frac{1}{2} \times \frac{1}{4} = \frac{1}{8}

Evaluating the left-hand side integral: [x22−x44]0α=α22−α44\left[ \frac{x^2}{2} - \frac{x^4}{4} \right]_{0}^{\alpha} = \frac{\alpha^2}{2} - \frac{\alpha^4}{4}

Equating this to 18\frac{1}{8}: α22−α44=18\frac{\alpha^2}{2} - \frac{\alpha^4}{4} = \frac{1}{8}

Multiply the entire equation by 88: 4α2−2α4=1  ⟹  2α4−4α2+1=04\alpha^2 - 2\alpha^4 = 1 \implies 2\alpha^4 - 4\alpha^2 + 1 = 0

This verifies option (C).


Step 3: Solve for α\alpha and Determine its Range

Let t=α2t = \alpha^2. Substituting tt into the equation gives a quadratic equation in tt: 2t2−4t+1=02t^2 - 4t + 1 = 0

Solving for tt using the quadratic formula: t=−(−4)±(−4)2−4(2)(1)2(2)=4±84=1±12t = \frac{-(-4) \pm \sqrt{(-4)^2 - 4(2)(1)}}{2(2)} = \frac{4 \pm \sqrt{8}}{4} = 1 \pm \frac{1}{\sqrt{2}}

Since 0<α<10 < \alpha < 1, we must have 0<t<10 < t < 1. Thus, we choose the smaller root: α2=1−12\alpha^2 = 1 - \frac{1}{\sqrt{2}}

Now, let's compare α2\alpha^2 with 14\frac{1}{4}: 1−12=2−12=1−22≈1−0.7071=0.29291 - \frac{1}{\sqrt{2}} = \frac{\sqrt{2} - 1}{\sqrt{2}} = 1 - \frac{\sqrt{2}}{2} \approx 1 - 0.7071 = 0.2929

Since 0.2929>0.25=140.2929 > 0.25 = \frac{1}{4}: α2>14  ⟹  α>12\alpha^2 > \frac{1}{4} \implies \alpha > \frac{1}{2}

Similarly, since 1−12<11 - \frac{1}{\sqrt{2}} < 1: α2<1  ⟹  α<1\alpha^2 < 1 \implies \alpha < 1

Combining these bounds gives: 12<α<1\frac{1}{2} < \alpha < 1

This verifies option (B).


Conclusion:

The correct statements are (B) and (C).