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Question: Answered & Verified by Expert
A point that lies on the common tangent to the circles x2+y2-2x+18y+78=0 and x2+y2+8x-6y-200=0 among the following options is
MathematicsCircleTS EAMCETTS EAMCET 2021 (05 Aug Shift 2)
Options:
  • A 0, 13912
  • B -1375, -16
  • C 31, -43
  • D -25, -474
Solution:
1215 Upvotes Verified Answer
The correct answer is: -25, -474

Given circles are

S1x2+y2-2x+18y+78=0......1

Centre C11,-9 & radius r1=-12+92-78=2 units

And x2+y2+8x-6y-200=0......2

Centre C2=-4,3 & radius r2=42+-32+200=15 units

C1C2=-4-12+3+92=13=r2-r1

Circles touch each other internally.

Equation of common tangent to both the circles is given by S1-S2=0

x2+y2-2x+18y+78-x2+y2+8x-6y-200=0

-10x+24y+278=0

-5x+12y+139=0......3

Option 1

-5×0+12×13912+139=2780

0,1392 does not satisfy the equation 3, so 0,1392 does not lie on the common tangent.

Option 2

 -5×-1375+12×-16+139=2740

-1375,-16  does not lie on the common tangent.

Option 3

-5×31+12×-43+139=-220

31,-43 does not lie on the common tangent.

Option 4

-5×-25+12×-474+139=0

-25,-474 lies on the common tangent.

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