In fig. 3, a square OABC is inscribed in a quadrant OPBQ of a circle. If $OA\ =\ 20\ cm$, find the area of the shaded region $( Use\ \pi =\ 3.14)$.
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Given: A square OABC inscribing a quadrant OPBQ. $OA=20\ cm$.
To do: To find the area of shaded region.
Solution:
Here as shown in fig, OABC is a square and $OA=AB=BC=CA=20\ cm$
let us join OB.
now in $\vartriangle OAB$,
$OB^{2} =OA^{2} +AB^{2}$
$\Rightarrow \ OB^{2} =20^{2} +20^{2}$
$\Rightarrow OB^{2} =400+400$
$\Rightarrow OB=\sqrt{800}$
$\Rightarrow OB=20\sqrt{2}$
Here OB is the radius r of the given quadrant.
Area of the given quadrant, $A_{1} =\frac{\theta }{360^{o}} \times \pi r^{2}$
$A_{1} =\frac{90^{o}}{360^{o} } \times 3.14\times \left( 20\sqrt{2}\right)^{2}$
$\Rightarrow A_{1} =\frac{1}{4} \times 3.14\times 800$
$\Rightarrow A_{1} =628cm^{2}$
Area of the square $A_{2} =( side)^{2}$
$\Rightarrow A_{2} =20\times 20$
$\Rightarrow A_{2} =400cm^{2}$
Area of the shaded region, $A=$Area of the quadrant, $A_{1} -$Area of the square, $A_{2}$
$A=628-400$
$\Rightarrow A=228cm^{2}$
Therefore, area of the shaded region is $228cm^{2}$ .
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