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Circle question

2003 · Shift 0 · Q113
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Circle question

2003 · Shift 0 · Q113

JEE MainMathematicsCircleMCQ+4 / −1
The lines 2x - 3y = 5 and 3x - 4y = 7 are diameters of a circle having area as 154 sq. units. Then the equation of the circle is :
  1. A
    x2 + y2− 2x + 2y  = 62{x^2}\, + \,{y^2} - \,2x\, + \,2y\,\, = \,62x2+y2−2x+2y=62
  2. B
    x2 + y2+ 2x − 2y  = 62{x^2}\, + \,{y^2} + \,2x\, - \,2y\,\, = \,62x2+y2+2x−2y=62
  3. C
    x2 + y2+ 2x − 2y  = 47{x^2}\, + \,{y^2} + \,2x\, - \,2y\,\, = \,47x2+y2+2x−2y=47
  4. D
    x2 + y2− 2x + 2y  = 47{x^2}\, + \,{y^2} - \,2x\, + \,2y\,\, = \,47x2+y2−2x+2y=47
View written solutionFree

Correct answer: D

  1. Key idea: Diameters pass through the center

    Since the given lines 2x−3y=52x - 3y = 52x−3y=5 and 3x−4y=73x - 4y = 73x−4y=7 are diameters of the circle, both lines must pass through the center of the circle.

    So, the center is the point of intersection of these two lines.

  2. Find the center by solving the two equations

    We solve: 2x−3y=5...(1)2x - 3y = 5 \quad ...(1)2x−3y=5...(1) 3x−4y=7...(2)3x - 4y = 7 \quad ...(2)3x−4y=7...(2)

    Multiply (1) by 333: 6x−9y=156x - 9y = 156x−9y=15

    Multiply (2) by 222: 6x−8y=146x - 8y = 146x−8y=14

    Subtract: (6x−8y)−(6x−9y)=14−15 (6x-8y) - (6x-9y) = 14-15(6x−8y)−(6x−9y)=14−15 y=−1y = -1y=−1

    Substitute into (1): 2x−3(−1)=52x - 3(-1) = 52x−3(−1)=5 2x+3=52x + 3 = 52x+3=5 2x=22x = 22x=2 x=1x = 1x=1

    Hence, the center is C(1,−1).C(1,-1).C(1,−1).

  3. Use the area to find the radius

    Area of the circle is given as 154154154 square units.

    πr2=154\pi r^2 = 154πr2=154

    Using π=227,\pi = \frac{22}{7},π=722​, r2=154π=15422/7=154⋅722=49r^2 = \frac{154}{\pi} = \frac{154}{22/7} = 154 \cdot \frac{7}{22} = 49r2=π154​=22/7154​=154⋅227​=49

    So, r=7.r = 7.r=7.

  4. Write the equation of the circle

    Standard form: (x−h)2+(y−k)2=r2(x-h)^2 + (y-k)^2 = r^2(x−h)2+(y−k)2=r2

    Here, (h,k)=(1,−1)(h,k) = (1,-1)(h,k)=(1,−1) and r2=49r^2 = 49r2=49.

    Therefore, (x−1)2+(y+1)2=49(x-1)^2 + (y+1)^2 = 49(x−1)2+(y+1)2=49

  5. Expand the equation

    x2−2x+1+y2+2y+1=49x^2 - 2x + 1 + y^2 + 2y + 1 = 49x2−2x+1+y2+2y+1=49 x2+y2−2x+2y+2=49x^2 + y^2 - 2x + 2y + 2 = 49x2+y2−2x+2y+2=49 x2+y2−2x+2y=47x^2 + y^2 - 2x + 2y = 47x2+y2−2x+2y=47

  6. Match with the options

    This matches: x2+y2−2x+2y=47\boxed{x^2 + y^2 - 2x + 2y = 47}x2+y2−2x+2y=47​

    So, the correct option is D.

  7. Comparison with stored answer

    Stored correct answer: D

    Our derived answer is also D, so they agree.

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