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Circle question

2024 · 8 Apr · Shift 1 · Q35
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  5. /2024 · 8 Apr · Shift 1 · Q35

Circle question

2024 · 8 Apr · Shift 1 · Q35

JEE MainMathematicsCircleMCQ+4 / −1
Let the circles C1:(x−α)2+(y−β)2=r12C_1:(x-\alpha)^2+(y-\beta)^2=r_1^2C1​:(x−α)2+(y−β)2=r12​ and C2:(x−8)2+(y−152)2=r22C_2:(x-8)^2+\left(y-\frac{15}{2}\right)^2=r_2^2C2​:(x−8)2+(y−215​)2=r22​ touch each other externally at the point (6,6)(6,6)(6,6). If the point (6,6)(6,6)(6,6) divides the line segment joining the centres of the circles C1C_1C1​ and C2C_2C2​ internally in the ratio 2:12: 12:1, then (α+β)+4(r12+r22)(\alpha+\beta)+4\left(r_1^2+r_2^2\right)(α+β)+4(r12​+r22​) equals
  1. A
    130
  2. B
    110
  3. C
    145
  4. D
    125
View written solutionFree

Correct answer: A

  1. Identify the centres of the circles

For C1:(x−α)2+(y−β)2=r12,C_1:(x-\alpha)^2+(y-\beta)^2=r_1^2,C1​:(x−α)2+(y−β)2=r12​, its centre is O1=(α,β).O_1=(\alpha,\beta).O1​=(α,β).

For C2:(x−8)2+(y−152)2=r22,C_2:(x-8)^2+\left(y-\frac{15}{2}\right)^2=r_2^2,C2​:(x−8)2+(y−215​)2=r22​, its centre is O2=(8,152).O_2=\left(8,\frac{15}{2}\right).O2​=(8,215​).

The circles touch externally at P=(6,6).P=(6,6).P=(6,6).


  1. Use the ratio condition

The point P=(6,6)P=(6,6)P=(6,6) divides the line segment joining the centres O1O_1O1​ and O2O_2O2​ internally in the ratio 2:12:12:1.

This means O1P:PO2=2:1.O_1P:PO_2=2:1.O1​P:PO2​=2:1.

Using section formula, if PPP divides O1O2O_1O_2O1​O2​ internally in the ratio 2:12:12:1, then P=(2x2+1x13,2y2+1y13)P=\left(\frac{2x_2+1x_1}{3},\frac{2y_2+1y_1}{3}\right)P=(32x2​+1x1​​,32y2​+1y1​​) where (x1,y1)=O1=(α,β)(x_1,y_1)=O_1=(\alpha,\beta)(x1​,y1​)=O1​=(α,β) and (x2,y2)=O2=(8,152)(x_2,y_2)=O_2=\left(8,\frac{15}{2}\right)(x2​,y2​)=O2​=(8,215​).

So, 6=2⋅8+α36=\frac{2\cdot 8+\alpha}{3}6=32⋅8+α​ 18=16+α18=16+\alpha18=16+α α=2.\alpha=2.α=2.

Also, 6=2⋅152+β36=\frac{2\cdot \frac{15}{2}+\beta}{3}6=32⋅215​+β​ 18=15+β18=15+\beta18=15+β β=3.\beta=3.β=3.

Hence, α+β=2+3=5.\alpha+\beta=2+3=5.α+β=2+3=5.


  1. Use the touching condition to find radii

Since the circles touch externally at PPP, the point PPP lies on both circles, and the distances from centres to PPP are the radii.

Thus, r1=O1P,r2=O2P.r_1=O_1P,\qquad r_2=O_2P.r1​=O1​P,r2​=O2​P.

For C1C_1C1​:

Centre O1=(2,3)O_1=(2,3)O1​=(2,3), point P=(6,6)P=(6,6)P=(6,6).

r12=(6−2)2+(6−3)2=42+32=16+9=25.r_1^2=(6-2)^2+(6-3)^2=4^2+3^2=16+9=25.r12​=(6−2)2+(6−3)2=42+32=16+9=25. So, r1=5.r_1=5.r1​=5.

For C2C_2C2​:

Centre O2=(8,152)O_2=\left(8,\frac{15}{2}\right)O2​=(8,215​), point P=(6,6)P=(6,6)P=(6,6).

r22=(6−8)2+(6−152)2r_2^2=(6-8)^2+\left(6-\frac{15}{2}\right)^2r22​=(6−8)2+(6−215​)2 =(−2)2+(−32)2=4+94=254.=(-2)^2+\left(-\frac{3}{2}\right)^2=4+\frac{9}{4}=\frac{25}{4}.=(−2)2+(−23​)2=4+49​=425​.

So, r2=52.r_2=\frac{5}{2}.r2​=25​.


  1. Compute the required expression

We need (α+β)+4(r12+r22).(\alpha+\beta)+4(r_1^2+r_2^2).(α+β)+4(r12​+r22​).

Substitute the values: α+β=5,\alpha+\beta=5,α+β=5, r12=25,r22=254.r_1^2=25,\qquad r_2^2=\frac{25}{4}.r12​=25,r22​=425​.

Then r12+r22=25+254=100+254=1254.r_1^2+r_2^2=25+\frac{25}{4}=\frac{100+25}{4}=\frac{125}{4}.r12​+r22​=25+425​=4100+25​=4125​.

Therefore, 4(r12+r22)=4⋅1254=125.4(r_1^2+r_2^2)=4\cdot \frac{125}{4}=125.4(r12​+r22​)=4⋅4125​=125.

Hence, (α+β)+4(r12+r22)=5+125=130.(\alpha+\beta)+4(r_1^2+r_2^2)=5+125=130.(α+β)+4(r12​+r22​)=5+125=130.


  1. Check with options

The value is 130.130.130. So the correct option is A.


  1. Compare with stored correct answer

Stored correct answer: A

Our derived answer: A

They agree.

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