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CBSE Class 12 Math 2013 Solved Paper

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Question : 25 of 29
Marks: +1, -0
Using integration, find the area bounded by the curve x2x^2 = 4y and the line x = 4y – 2.
OR
Using integration, find the area of the region enclosed between the two circles
x2+y2x^2 + y^2 = 4 and (x2)2+y2(x - 2)^2 + y^2 = 4.
Solution:  
The shaded area OBAO represents the area bounded by the curve x2x^2 = 4y and line x = 4y – 2.
Let A and B be the points of intersection of the line and parabola.
Co-ordinates of point A are (1,14)\left(-1,\frac{1}{4}\right). Co-ordinates of point B are (2, 1).
Area OBAO = Area OBCO + Area OACO … (1)
Area OBCO = 02x+24\int\limits_{0}^{2} \frac{x+2}{4} dx - 02x24\int\limits_{0}^{2} \frac{x^2}{4} dx
= 14[x22+2x]02\frac{1}{4}\left[ \frac{x^2}{2} + 2x \right]_{0}^{2} - 14[x33]02\frac{1}{4}\left[ \frac{x^3}{3} \right]_{0}^{2}
= 12\frac{1}{2} (2 + 4) - 14[83]\frac{1}{4} \left[ \frac{8}{3} \right]
= 3223\frac{3}{2} - \frac{2}{3} = 56\frac{5}{6}
Area OACO = 10x+24\int\limits_{-1}^{0} \frac{x+2}{4} dx - 10x24\int\limits_{-1}^{0} \frac{x^2}{4} dx
= 14x2+2+2x10\frac{1}{4}\left| x^2 + 2 + 2x \right|_{-1}^{0} - 14x3310\frac{1}{4}\left| \frac{x^3}{3} \right|_{-1}^{0}
= 14(1)2221\frac{1}{4}\left| \frac{(-1)^2}{2} - 2 - 1 \right| - 14(13)3\frac{1}{4}\left| -\left( -\frac{1}{3} \right)^3 \right|
= 1412+2\frac{1}{4}\left| -\frac{1}{2} + 2 \right| - 14(13)\frac{1}{4} \left( \frac{1}{3} \right)
= 38112\frac{3}{8} - \frac{1}{12} = 724\frac{7}{24}
Therefore, required area = (56+724)\left( \frac{5}{6} + \frac{7}{24} \right) = 98\frac{9}{8} sq. units
OR
Given equations of the circles are
x2+y2x^2 + y^2 = 4 ... (1)
(x2)2+y2(x-2)^2 + y^2 = 4 ... (2)
Equation (1) is a circle with centre O at the origin and radius 2. Equation (2) is a circle with centre C (2, 0) and radius 2.
Solving (1) and (2), we have:
(x2)2+y2(x-2)^2 + y^2 = x1+y2x^1 + y^2
x2x^2 - 4x + 4 + y2y^2 = x2+y2x^2 + y^2
x = 1
This gives y = ±3\pm \sqrt{3}
Thus, the points of intersection of the given circles are A (1 , 3)\sqrt{3}) and A' (1 , - 3\sqrt{3}) as shown in the figure.
Required area
= Area of the region OACA'O
= 2 [area of the region ODCAO]
= 2 [area of the region ODAO + area of the region DCAD]
= 2 [01ydx+12ydx]\left[ \int\limits_{0}^{1} y\,dx + \int\limits_{1}^{2} y\,dx \right]
= 2 [014(x2)2\int\limits_{0}^{1} \sqrt{4-(x-2)^2} dx + 124x2\int\limits_{1}^{2} \sqrt{4-x^2} dx]
= 2
[12(x2)4(x2)2+4sin1(x22)]01\left[ \frac{1}{2} (x-2) \sqrt{4-(x-2)^2} + 4 \sin^{-1}\left(\frac{x-2}{2}\right) \right]_{0}^{1}
+ [x4x2+4sin1(x2)]12\left[ x \sqrt{4-x^2} + 4 \sin^{-1}\left(\frac{x}{2}\right) \right]_{1}^{2}
=
[(3+4sin1(12))4sin1(1)]\left[ \left( -\sqrt{3} + 4 \sin^{-1}\left(\frac{-1}{2}\right) \right) - 4 \sin^{-1}(-1) \right]
+ [4sin1134sin1(12)]\left[ 4 \sin^{-1} 1 - \sqrt{3} - 4 \sin^{-1}\left(\frac{1}{2}\right) \right]
= [(34×π6)+4×π2]\left[ \left( -\sqrt{3} - 4 \times \frac{\pi}{6} \right) + 4 \times \frac{\pi}{2} \right] + [4×π234×π6]\left[ 4 \times \frac{\pi}{2} - \sqrt{3} - 4 \times \frac{\pi}{6} \right]
= 8π3\frac{8\pi}{3} - 2 3\sqrt{3}
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