In the figure below, the rectangle at the corner measures 10cm×20cm. The corner A of the rectangle is also a point on the circumference of the circle. What is the radius of the circle in cm?
A
10 cm
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B
40 cm
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C
50 cm
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D
30 cm
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Solution
The correct option is C 50 cm WetakethecornerOofthegivensquareOPQR,containingthe10cm×20cmrectangle,astheorigineandthesidesOP&ORasthecoordinateaxes.LetthesidesofOPQRbe2a.SoOPQRliesinthethirdquadrant.Theinscribecircle′sradiusisr=a¢reisC(a,a).ThecirclepassesthroughA(−10,−20).∴Theequationofthecircleis(x−h)2+(y−k)2=r2..........(i)Here(h,k)=(a,a)andr=a∴equation(i)becomes(x−a)2+(y−a)2=a2..........(ii)ThecirclepassesthroughA(−10,−20).i.e(x,y)=(−10,−20)Soequation(ii)reducesto(−10−a)2+(−20−a)2=a2.Onsolvingforaweget|a|=50unitsand10units.(aisalength.Sowedonotconsiderthenegativesign).AlsoA(−10,−20)lieonthecircle.midwaybetween(a,0)&(0,a).Soa>10units&a>20units.∴werejecta=10.soa=50unitsAns−OptionC