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Grade 11Algebra

Let ABC be a triangle with indenter I and inradius r. Let D, E, F be the feet of the perpendiculars from I to the sides BC, CA and AB respectively. If r1, r2 and r3 are the radii of circles inscribed in the quadrilaterals AFIE, BDIF and CEID respectively, prove that
r1/r – r1 + r2/r - r2 + r3/r – r3 = r1 r2 r3/(r – r1) (r – r2) (r – r3).

Profile image of Radhika Batra
12 Years agoGrade 11
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1 Answer

Profile image of Jitender Pal
12 Years ago
Hello Student,
Please find the answer to your question
Let MN = r3 = MP = MQ, ID = r
⇒ IP = r – r3
Clearly IP and IQ are tangents to circle with centre M.
∴ IM must be the ∠ bisector of ∠ PIQ
∴ ∠PIM = ∠QIM = θ1
Also from ∆ IPM, tan θ1 = r3/r – r3 = MP/IP
235-1694_12345.png
Similarly, in other quadrilaterals, we get
tan θ2 = r2/r – r2 and tan θ3 = r1/r – r1
Also 2θ1 + 2θ2 + 2θ3 = 2π ⇒ θ1 + θ2 + θ3 = π
⇒ tan θ1 tan θ2 + tan θ1 θ3 = tan θ1. tan θ2. tan θ3
NOTE THIS STEP:
= r1/r – r1 + r2/r – r2 + r/r – r3 = r1 r2 r3/(r – r1) (r – r2) (r – r3)

Thanks
Jitender Pal
askIITians Faculty