Consider a circle C_1:x^2+y^2–4x–2y=α –5. Let its mirror image in the line y=2x+1 be another circle C_2:5x^2+5y^2–10fx–10gy+36=0. Let r be the radius…
JEE Main 2023 — Mathematics Coordinate Geometry
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Consider a circle C1:x2+y2–4x–2y=α–5. Let its mirror image in the line y=2x+1 be another circle C2:5x2+5y2–10fx–10gy+36=0. Let r be the radius of C2 . Then α+r is equal to ________
Official previous-year question
Held on 8 Apr 2023 · Verified 6 Jul 2026.
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Solution
Given,
C1:x2+y2–4x–2y+(5–α)=0
So, its centre will be, O1=(2,1) and radius =α
And C2:5x2+5y2–10fx–10gy+36=0
⇒C2:x2+y2–2fx–2gy+536=0
So, Centre O2=(f,g) and radius r=f2+g2−536
Also given O2 is reflection of O1 in 2x–y+1=0, so image formula we get,
⇒2f−2=−1g−1=−2⋅(22+122×2−1+1)
⇒f=5−6 and g=513
So, radius r=(5−6)2+(513)2−536=525=1
⇒r=1and α=1 as they both are same radius circle,
Hence, r+α=2.
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