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← JEE Main 2014 · 11 Apr · Shift 1
MathematicsSingle correct+4 / -1Question 80 of 90

JEE Main 2014 · 11 Apr · Shift 1, Mathematics question 80

The set of all real values of λ\lambda for which exactly two common tangents can be drawn to the circles x2+y2−4x−4y+6=0x^2+y^2-4x-4y+6=0 and x2+y2−10x−10y+λ=0x^2+y^2-10x-10y+\lambda=0 is the interval :

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Answer and worked solution

Official answer: B

The circles are C1(2,2)C_1(2,2), r1=2r_1=\sqrt2 and C2(5,5)C_2(5,5), r2=50−λr_2=\sqrt{50-\lambda}, with d=32d=3\sqrt2.

Exactly two common tangents means they intersect: ∣r2−r1∣<d<r1+r2|r_2-r_1|<d<r_1+r_2. This gives 22<r2<422\sqrt2<r_2<4\sqrt2, so 8<50−λ<328<50-\lambda<32 and λ∈(18,42)\lambda\in(18,42).