Which area is larger?

Geometry Level 3

Line segment D C DC is tangent to the circle with center B B at A A , A D = 3 B E AD= \sqrt{3} BE .

Which area is larger, green or red?

green red Relationship can't be determined Both are equal

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1 solution

Hana Wehbi
Jun 16, 2017

D A B = 90 \angle DAB= 90 degrees since D C DC is tangent to the circle and D B A = 60 \angle DBA= 60 degrees, we can verify that by trigonometry:

tan D B A = o p p a d j = 3 r r = 3 \tan \angle DBA= \frac{opp}{adj}= \frac {\sqrt{3}r}{r} = \sqrt{3} , which implies that D B A = 60 \angle DBA= 60 degrees.

Let r r be the radius of the circle and A B = B E = r AB = BE= r , then the area of triangle A B C ABC is:

A = 1 2 b h = ( 3 ) 2 r 2 A = \frac{1}{2}bh = \frac{\sqrt(3)}{2} r^2 .

The area of the green region is 1 6 π r 2 \frac{1}{6}\pi r^2 . Area of a sector in a circle 60 360 π r 2 \frac{60}{360}\pi r^2 .

The area of the red region is 3 2 r 2 π 6 r 2 \frac{\sqrt{3}}{2}r^2- \frac{\pi}{6}r^2 = Area of a triangle - Area of the green sector.

Since π 6 r 2 > 3 2 r 2 π 6 r 2 \frac{\pi}{6}r^2 > \frac{\sqrt{3}}{2}r^2 - \frac{\pi}{6}r^2 . Take π 6 = 0.524 and 3 2 π 6 = 0.342 \frac{\pi}{6} = 0.524 \ \text{and} \ \frac{\sqrt{3}}{2} - \frac{\pi}{6} = 0.342 .

which means that π 6 > 3 2 π 6 \frac{\pi}{6} > \frac{\sqrt{3}}{2} - \frac{\pi}{6} and π 6 r 2 > ( 3 2 π 6 ) r 2 \frac{\pi}{6} r^2 > (\frac{\sqrt{3}}{2} - \frac{\pi}{6}) r^2 .

This means that the green area is larger.

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