Area calculation

Geometry Level 3

The circle above has its center at point O O . The length of A B AB is 25 feet. If the radius of the circle is 17 feet, which of the following is the nearest area of the yellow region in square feet?

192 f t 2 192~ft^2 120 f t 2 120~ft^2 312 f t 2 312~ft^2 102 f t 2 102~ft^2 182 f t 2 182~ft^2

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1 solution

The yellow region is a segment of a circle. Draw O C OC and O D OD to form a sector of a circle. Apply pythagorean theorem in C A O \triangle CAO .

( C A ) 2 = 1 7 2 8 2 (CA)^2=17^2-8^2

( C A ) 2 = 225 (CA)^2=225

C A = 225 CA=\sqrt{225}

C A = 15 CA=15 feet \text{feet}

It follows that,

C D = 2 ( C A ) = 2 ( 15 ) = 30 CD=2(CA)=2(15)=30 feet \text{feet}

Apply cosine law in C D O \triangle CDO ,

3 0 2 = 1 7 2 + 1 7 2 2 ( 17 ) ( 17 ) ( c o s C O D ) 30^2=17^2+17^2-2(17)(17)(cos~\angle COD)

c o s ( C O D ) = 161 289 cos~(\angle COD)=-\frac{161}{289}

C O D = c o s 1 ( 161 289 ) 123.85 5 \angle COD=cos^{-1}(-\frac{161}{289})\approx 123.855^\circ

Calculate the area of the sector:

A s e c t o r = C O D 360 ( π ) ( r 2 ) = 123.855 360 ( 3.14 ) ( 1 7 2 ) 312.2 f t 2 A_{sector}=\frac{\angle COD}{360}(\pi)(r^2)=\frac{123.855}{360}(3.14)(17^2)\approx 312.2~ft^2

Calculate the area of the triangle:

A T = 1 2 ( b ) ( h ) = 1 2 ( 30 ) ( 8 ) = 120 f t 2 A_{T}=\frac{1}{2}(b)(h)=\frac{1}{2}(30)(8)=120`ft^2

Calculate the area of the segment:

A = A s e c t o r A T = 312.2 120 = 192.2 f t 2 A=A_{sector}-A_{T}=312.2-120=192.2~ft^2

Based from the given options, the nearest area of the yellow region is 192 f t 2 192~ft^2 .

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