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Question

In a Δ ABC, R = circumradius and r = inradius. The value of acosA+bcosB+ccosCa+b+c is equal to


A

R/r

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B

R/2r

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C

r/R

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D

2 r/R

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Solution

The correct option is C

r/R


acosA+bcosB+ccosCa+b+c=2RsinAcosA+2RsinBcosB+2rsinCcosC2s=R2s.(sin2A+sin2B+sin2C)=R2s.4sinAsinBsinC=R2s.4abc8R3=abc4sR2But R=abc4Δ,r=ΔsSo, the value=4ΔR4.Δr.R2=rR


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