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Circle question

2023 · 6 Apr · Shift 1 · Q41
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  5. /2023 · 6 Apr · Shift 1 · Q41

Circle question

2023 · 6 Apr · Shift 1 · Q41

JEE MainMathematicsCircleNumerical+4 / −1
A circle passing through the point P(α,β)P(\alpha, \beta)P(α,β) in the first quadrant touches the two coordinate axes at the points AAA and BBB. The point PPP is above the line ABA BAB. The point QQQ on the line segment ABA BAB is the foot of perpendicular from PPP on ABA BAB. If PQP QPQ is equal to 11 units, then the value of αβ\alpha \betaαβ is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 121

  1. Set up the circle

Since the circle touches both coordinate axes, its center must be at equal distance from both axes. So let the center be C(r,r)C(r,r)C(r,r) with radius rrr.

Therefore, the points of contact with the axes are: A(r,0),B(0,r)A(r,0), \quad B(0,r)A(r,0),B(0,r) So the line ABABAB passes through (r,0)(r,0)(r,0) and (0,r)(0,r)(0,r).

Its equation is x+y=rx+y=rx+y=r

  1. Use the condition that P(α,β)P(\alpha,\beta)P(α,β) lies on the circle

The circle equation is (x−r)2+(y−r)2=r2(x-r)^2+(y-r)^2=r^2(x−r)2+(y−r)2=r2 Since P(α,β)P(\alpha,\beta)P(α,β) lies on it, (α−r)2+(β−r)2=r2(\alpha-r)^2+(\beta-r)^2=r^2(α−r)2+(β−r)2=r2 Expanding, α2−2αr+r2+β2−2βr+r2=r2\alpha^2-2\alpha r+r^2+\beta^2-2\beta r+r^2=r^2α2−2αr+r2+β2−2βr+r2=r2 α2+β2−2r(α+β)+r2=0...(1) \alpha^2+\beta^2-2r(\alpha+\beta)+r^2=0 \quad ...(1)α2+β2−2r(α+β)+r2=0...(1)

  1. Use the distance from PPP to line ABABAB

Given that QQQ is the foot of perpendicular from PPP to ABABAB, we have PQ=distance from (α,β) to x+y−r=0PQ = \text{distance from }(\alpha,\beta)\text{ to }x+y-r=0PQ=distance from (α,β) to x+y−r=0 So, PQ=∣α+β−r∣2PQ=\frac{|\alpha+\beta-r|}{\sqrt{2}}PQ=2​∣α+β−r∣​ Given PQ=11PQ=11PQ=11 and PPP is above the line ABABAB, hence α+β−r>0\alpha+\beta-r>0α+β−r>0 Therefore, α+β−r2=11\frac{\alpha+\beta-r}{\sqrt{2}}=112​α+β−r​=11 α+β−r=112\alpha+\beta-r=11\sqrt{2}α+β−r=112​ r=α+β−112...(2)r=\alpha+\beta-11\sqrt{2} \quad ...(2)r=α+β−112​...(2)

  1. Substitute into the circle condition

From (1): α2+β2−2r(α+β)+r2=0\alpha^2+\beta^2-2r(\alpha+\beta)+r^2=0α2+β2−2r(α+β)+r2=0 Let s=α+βs=\alpha+\betas=α+β Then α2+β2=s2−2αβ\alpha^2+\beta^2=s^2-2\alpha\betaα2+β2=s2−2αβ So (1) becomes s2−2αβ−2rs+r2=0s^2-2\alpha\beta-2rs+r^2=0s2−2αβ−2rs+r2=0 2αβ=s2−2rs+r2=(s−r)22\alpha\beta=s^2-2rs+r^2=(s-r)^22αβ=s2−2rs+r2=(s−r)2 Hence, 2αβ=(α+β−r)22\alpha\beta=(\alpha+\beta-r)^22αβ=(α+β−r)2 But from the distance condition, α+β−r=112\alpha+\beta-r=11\sqrt{2}α+β−r=112​ So, 2αβ=(112)2=121⋅22\alpha\beta=(11\sqrt{2})^2=121\cdot 22αβ=(112​)2=121⋅2 Thus, αβ=121\alpha\beta=121αβ=121

  1. Final answer

121\boxed{121}121​

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