In the given figure, quadrilateral ABCD is circumscribed and AD⊥AB. If radius of incircle is 10cm, then the value of x is
A
20cm
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B
42cm
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C
19cm
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D
21cm
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Solution
The correct option is D 21cm Given−Oisthecentreofacircle,inscribedinaquadrilateralABCD.Theradiusofthecircleis10cm.∠BAD=90o.AB,BC,CD&ADtouchtheinscribedcircleatP,Q,R&Srespectively.CR=27cmandBC=38cm.Tofindout−AB=x=?Solution−WejoinOS&OP.ThenOS&OPareradiioftheinscribedcirclethroughthepointsofcontactofthetangentsAS&APrespectively.∴OS=OP=10cm.Again∠OSA=90o=∠OPAsincetheradiusthroughthepointofcontactofatangenttoacircleisperpendiculartothetangent.NowinOSAP,∠OSA=90o=∠OPAandOS=OP=10cm.∴OSAPisasquareofside10cm.SoAP=OS=10cm.AgainCR=CQ=27cmandBQ=BPsincethelengthsofthetangents,fromapointtoacircle,areequal.∴BQ=BC−CQ=(38−27)cm=11cm.AlsoBP=BQ=11cm.∴AB=x=AP+BP=(10+11)cm=21cm.Ans−OptionD.