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Circle question

2012 · Shift 0 · Q38
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Circle question

2012 · Shift 0 · Q38

JEE MainMathematicsCircleMCQ+4 / −1
The length of the diameter of the circle which touches the xxx-axis at the point (1,0)(1, 0)(1,0) and passes through the point (2,3)(2, 3)(2,3) is :
  1. A
    103{{10} \over 3}310​
  2. B
    35{{3} \over 5}53​
  3. C
    65{{6} \over 5}56​
  4. D
    53{{5} \over 3}35​
View written solutionFree

Correct answer: A

  1. Use the condition of touching the xxx-axis at (1,0)(1,0)(1,0)

    If a circle touches the xxx-axis at (1,0)(1,0)(1,0), then the radius at the point of contact is perpendicular to the xxx-axis.

    Hence the center must lie vertically above or below (1,0)(1,0)(1,0).

    So let the center be C=(1,r)C=(1,r)C=(1,r) where rrr is the radius. Since the circle touches the xxx-axis, its radius is exactly the distance from the center to the xxx-axis, which is ∣r∣|r|∣r∣. Taking the center above the axis, C=(1,r),radius=r.C=(1,r), \quad \text{radius}=r.C=(1,r),radius=r.

  2. Use the fact that the circle passes through (2,3)(2,3)(2,3)

    The distance from the center (1,r)(1,r)(1,r) to the point (2,3)(2,3)(2,3) must be equal to the radius rrr: (2−1)2+(3−r)2=r.\sqrt{(2-1)^2+(3-r)^2}=r.(2−1)2+(3−r)2​=r.

    Squaring both sides: 1+(3−r)2=r2.1+(3-r)^2=r^2.1+(3−r)2=r2.

  3. Simplify

    1+9−6r+r2=r21+9-6r+r^2=r^21+9−6r+r2=r2 10−6r=010-6r=010−6r=0 6r=106r=106r=10 r=53.r=\frac{5}{3}.r=35​.

  4. Find the diameter

    Diameter =2r=2r=2r: 2⋅53=103.2\cdot \frac{5}{3}=\frac{10}{3}.2⋅35​=310​.

  5. Check options

    • A: 103\frac{10}{3}310​ ✅
    • B: 35\frac{3}{5}53​
    • C: 65\frac{6}{5}56​
    • D: 53\frac{5}{3}35​

Therefore, the correct answer is A.

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