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Circle question

2023 · 30 Jan · Shift 2 · Q40
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  5. /2023 · 30 Jan · Shift 2 · Q40

Circle question

2023 · 30 Jan · Shift 2 · Q40

JEE MainMathematicsCircleNumerical+4 / −1
Let P(a1,b1)P\left(a_1, b_1\right)P(a1​,b1​) and Q(a2,b2)Q\left(a_2, b_2\right)Q(a2​,b2​) be two distinct points on a circle with center C(2,3)C(\sqrt{2}, \sqrt{3})C(2​,3​). Let O\mathrm{O}O be the origin and OC\mathrm{OC}OC be perpendicular to both CP\mathrm{CP}CP and CQ\mathrm{CQ}CQ. If the area of the triangle OCP\mathrm{OCP}OCP is 352\frac{\sqrt{35}}{2}235​​, then a12+a22+b12+b22a_1^2+a_2^2+b_1^2+b_2^2a12​+a22​+b12​+b22​ is equal to :
Numerical answer
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Correct answer: 24

  1. Given data

    Center of the circle is C(2,3).C(\sqrt{2},\sqrt{3}).C(2​,3​). Origin is O(0,0)O(0,0)O(0,0).

    Also, OCOCOC is perpendicular to both CPCPCP and CQCQCQ.

  2. Interpret the perpendicular condition

    Since PPP and QQQ lie on the circle centered at CCC, the vectors CP→\overrightarrow{CP}CP and CQ→\overrightarrow{CQ}CQ​ are radii.

    Given OC⊥CPandOC⊥CQ,OC \perp CP \quad \text{and} \quad OC \perp CQ,OC⊥CPandOC⊥CQ, both radii are perpendicular to the same line OCOCOC.

    Through point CCC, there are exactly two opposite radius directions perpendicular to OCOCOC. Hence PPP and QQQ are endpoints of a diameter perpendicular to OCOCOC.

    So if the radius is rrr, then CP=CQ=r.CP=CQ=r.CP=CQ=r.

  3. Find the length of OCOCOC

    OC=(2)2+(3)2=2+3=5.OC=\sqrt{(\sqrt{2})^2+(\sqrt{3})^2}=\sqrt{2+3}=\sqrt{5}.OC=(2​)2+(3​)2​=2+3​=5​.

  4. Use area of triangle OCPOCPOCP

    Since OC⊥CPOC \perp CPOC⊥CP, triangle OCPOCPOCP is right-angled at CCC.

    Therefore, \text{Area}(\triangle OCP)=\frac{1}{2}\cdot OC\cdot CP= rac{1}{2}\cdot \sqrt{5}\cdot r.

    Given area is 352.\frac{\sqrt{35}}{2}.235​​.

    Hence, 125 r=352\frac{1}{2}\sqrt{5}\,r=\frac{\sqrt{35}}{2}21​5​r=235​​ 5 r=35\sqrt{5}\,r=\sqrt{35}5​r=35​ r=355=7.r=\frac{\sqrt{35}}{\sqrt{5}}=\sqrt{7}.r=5​35​​=7​.

  5. Use midpoint relation for opposite endpoints of a diameter

    Since PPP and QQQ are diametrically opposite with center CCC, their midpoint is CCC.

    Thus, (a1+a22,b1+b22)=(2,3).\left(\frac{a_1+a_2}{2},\frac{b_1+b_2}{2}\right)=(\sqrt{2},\sqrt{3}).(2a1​+a2​​,2b1​+b2​​)=(2​,3​).

    Also, CP=CQ=r=7.CP=CQ=r=\sqrt{7}.CP=CQ=r=7​.

  6. Required quantity

    We need a12+a22+b12+b22=OP2+OQ2.a_1^2+a_2^2+b_1^2+b_2^2=OP^2+OQ^2.a12​+a22​+b12​+b22​=OP2+OQ2.

    Let OP⃗=OC⃗+CP⃗,OQ⃗=OC⃗+CQ⃗.\vec{OP}=\vec{OC}+\vec{CP}, \qquad \vec{OQ}=\vec{OC}+\vec{CQ}.OP=OC+CP,OQ​=OC+CQ​.

    Since PPP and QQQ are opposite endpoints of a diameter, CQ⃗=−CP⃗.\vec{CQ}=-\vec{CP}.CQ​=−CP.

    Therefore, OP2=∣OC⃗+CP⃗∣2=OC2+CP2+2OC⃗⋅CP⃗,OP^2=|\vec{OC}+\vec{CP}|^2=OC^2+CP^2+2\vec{OC}\cdot\vec{CP},OP2=∣OC+CP∣2=OC2+CP2+2OC⋅CP, OQ2=∣OC⃗−CP⃗∣2=OC2+CP2−2OC⃗⋅CP⃗.OQ^2=|\vec{OC}-\vec{CP}|^2=OC^2+CP^2-2\vec{OC}\cdot\vec{CP}.OQ2=∣OC−CP∣2=OC2+CP2−2OC⋅CP.

    Adding, OP2+OQ2=2OC2+2CP2.OP^2+OQ^2=2OC^2+2CP^2.OP2+OQ2=2OC2+2CP2.

    Now, OC2=5,CP2=r2=7.OC^2=5, \qquad CP^2=r^2=7.OC2=5,CP2=r2=7.

    So, OP2+OQ2=2(5)+2(7)=10+14=24.OP^2+OQ^2=2(5)+2(7)=10+14=24.OP2+OQ2=2(5)+2(7)=10+14=24.

  7. Final answer

    a12+a22+b12+b22=24.a_1^2+a_2^2+b_1^2+b_2^2=24.a12​+a22​+b12​+b22​=24.

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