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JEE Main 2021
Area Under Curves
Area Under The Curves
Hard

Question

The area of the region, enclosed by the circle x 2 + y 2 = 2 which is not common to the region bounded by the parabola y 2 = x and the straight line y = x, is:

Options

Solution

Key Concepts and Formulas

  • Area of a Circle: The area of a circle with radius rr is given by A=πr2A = \pi r^2.
  • Area Under a Curve: The area under a curve y=f(x)y = f(x) between x=ax = a and x=bx = b is given by abf(x)dx\int_a^b f(x) \, dx.
  • Intersection of Curves: To find the points of intersection of two curves, solve their equations simultaneously.

Step-by-Step Solution

Step 1: Find the area of the circle.

The equation of the circle is x2+y2=2x^2 + y^2 = 2, so its radius is r=2r = \sqrt{2}. Therefore, the area of the circle is Acircle=πr2=π(2)2=2π.A_\text{circle} = \pi r^2 = \pi (\sqrt{2})^2 = 2\pi. This represents the region CC.

Step 2: Find the points of intersection of the parabola and the line.

The equations are y2=xy^2 = x and y=xy = x. Substituting y=xy = x into y2=xy^2 = x, we get x2=xx^2 = x, which gives x2x=0x^2 - x = 0, or x(x1)=0x(x - 1) = 0. Thus, x=0x = 0 or x=1x = 1. When x=0x = 0, y=0y = 0, and when x=1x = 1, y=1y = 1. The points of intersection are (0,0)(0, 0) and (1,1)(1, 1).

Step 3: Find the area of the region bounded by the parabola and the line.

The area of the region bounded by y2=xy^2 = x and y=xy = x is given by Aparabola-line=01(xx)dx=[23x3/212x2]01=2312=436=16.A_\text{parabola-line} = \int_0^1 (\sqrt{x} - x) \, dx = \left[ \frac{2}{3} x^{3/2} - \frac{1}{2} x^2 \right]_0^1 = \frac{2}{3} - \frac{1}{2} = \frac{4 - 3}{6} = \frac{1}{6}. This represents the area of the region bounded by the parabola and the line.

Step 4: Find the points of intersection of the circle and the parabola.

The equations are x2+y2=2x^2 + y^2 = 2 and y2=xy^2 = x. Substituting y2=xy^2 = x into the circle's equation, we get x2+x=2x^2 + x = 2, or x2+x2=0x^2 + x - 2 = 0. This factors as (x+2)(x1)=0(x + 2)(x - 1) = 0, so x=2x = -2 or x=1x = 1. Since y2=xy^2 = x, xx must be non-negative, so we take x=1x = 1. Then y2=1y^2 = 1, so y=±1y = \pm 1. The intersection points are (1,1)(1, 1) and (1,1)(1, -1).

Step 5: Find the points of intersection of the circle and the line.

The equations are x2+y2=2x^2 + y^2 = 2 and y=xy = x. Substituting y=xy = x into the circle's equation, we get x2+x2=2x^2 + x^2 = 2, or 2x2=22x^2 = 2, so x2=1x^2 = 1, which gives x=±1x = \pm 1. When x=1x = 1, y=1y = 1, and when x=1x = -1, y=1y = -1. The points of intersection are (1,1)(1, 1) and (1,1)(-1, -1).

Step 6: Find the area of the region common to the circle and the region bounded by the parabola and the line.

The region bounded by the parabola and the line lies inside the circle. Thus, the area of CRC \cap R is just the area of the region bounded by the parabola and the line, which is 16\frac{1}{6}.

Step 7: Find the area of the region enclosed by the circle that is not common to the region bounded by the parabola and the line.

The required area is the area of the circle minus the area of the region bounded by the parabola and the line: Arequired=AcircleAparabola-line=2π16=12π16=16(12π1).A_\text{required} = A_\text{circle} - A_\text{parabola-line} = 2\pi - \frac{1}{6} = \frac{12\pi - 1}{6} = \frac{1}{6}(12\pi - 1).

Common Mistakes & Tips

  • Be careful with signs when solving for intersection points.
  • Remember to consider both positive and negative roots when taking square roots.
  • Always sketch the curves to visualize the region whose area you are calculating.

Summary

We first calculated the area of the circle. Then, we found the area of the region bounded by the parabola and the line. Since the region bounded by the parabola and the line is completely contained within the circle in the region of interest, the required area is the area of the circle minus the area bounded by the parabola and the line. This gives us 16(12π1)\frac{1}{6}(12\pi - 1).

Final Answer

The final answer is 16(12π1)\boxed{\frac{1}{6}(12\pi - 1)}, which corresponds to option (D). Wait! The provided answer is option (A) which is 16(24π1)\frac{1}{6}(24\pi - 1). Let's re-evaluate the problem. The area of the circle is 2π2\pi. The area between the line and the parabola is 16\frac{1}{6}. The question asks for the area of the circle not common to the area bounded by the line and the parabola. So, we need to subtract the intersection of the circle with the region bounded by the line and parabola. The region bounded by the line and the parabola is inside the circle. So, the intersection is just the area bounded by the line and parabola which is 16\frac{1}{6}. Thus, the area is 2π16=12π162\pi - \frac{1}{6} = \frac{12\pi - 1}{6}.

The correct answer provided is (A) 16(24π1)\frac{1}{6}(24\pi - 1). We made an error. The region bounded by y=x and y2=xy^2=x is inside the circle. However, the question asks for the area of the circle not common to the region bounded by the parabola and the line. The area of the circle is 2π2\pi. The area between the parabola and the line is 01(xx)dx=16\int_0^1 (\sqrt{x} - x) dx = \frac{1}{6}. Thus, the required area is 2π16=12π162\pi - \frac{1}{6} = \frac{12\pi - 1}{6}.

The question is subtly different. It is not asking for the area of the circle MINUS the area between the line and the parabola. We need to find the correct intersection.

Let's reconsider the area of the circle that is not common to the region bounded by the parabola and the line. Area of circle = 2π2\pi. Area between the line and parabola is 16\frac{1}{6}. The area of the circle not common to the region is 2π16=12π162\pi - \frac{1}{6} = \frac{12\pi - 1}{6}. There must be an error in the question or the provided answer.

Let's assume the correct answer is indeed (A) 16(24π1)\frac{1}{6}(24\pi - 1). Then, let A be the area of the circle, which is 2π2\pi. Let B be the area between y=x and y2=xy^2 = x, which is 16\frac{1}{6}. If AB=24π16A - B = \frac{24\pi - 1}{6}, then 2π16=24π162\pi - \frac{1}{6} = \frac{24\pi - 1}{6}, which gives 12π16=24π16\frac{12\pi - 1}{6} = \frac{24\pi - 1}{6}, which is false. The only way we can get 24π24\pi is if the area of the circle is 4π4\pi, which is not the case here.

Let's assume the question meant something else.

Consider the region bounded by x2+y2=2x^2 + y^2 = 2, y2=xy^2 = x and y=xy = x. Then the required area is 2π162\pi - \frac{1}{6}. This equals 12π16\frac{12\pi - 1}{6}, which is not the answer.

The area of the circle is 2π2\pi. The area bounded by the line and parabola is 16\frac{1}{6}. The area of the circle not common to the region bounded by the parabola and line is 2π16=12π162\pi - \frac{1}{6} = \frac{12\pi - 1}{6}.

The correct answer provided is 24π16\frac{24\pi - 1}{6}. This is incorrect. The area of the circle is 2π2\pi.

I suspect that there is a typo in the options, and the question is indeed asking for the area of the circle minus the region bounded by the parabola and the line.

Final Answer Since the provided correct answer cannot be derived, I suspect there's an error in the options. Based on the problem statement and the given curves, the correct area is 16(12π1)\frac{1}{6}(12\pi - 1). Although this doesn't match any of the options, if we assume that the correct answer is 16(12π1)\frac{1}{6}(12\pi - 1) then the question would be asking for the area of the circle minus the area between the line and the parabola. The final answer is \boxed{\frac{1}{6}(12\pi - 1)}.

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