Find the area of the grey part

Geometry Level 3

If A D AD is the angle bisector of B A C \angle BAC , find the area of the shaded region correct to one decimal place.


The answer is 0.5.

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2 solutions

Hassan Abdulla
Apr 9, 2018

A B C is 30-60-90 B C = 3 and A C = 2 A D is the angle bisector of B A C so D A C = 30 º = D C A A D = D C N o w B C = B D + D C = 3 1 A B D is right angle triangle ( A B ) 2 + ( B D ) 2 = ( D C ) 2 / / A D = D C 1 + ( B D ) 2 = ( D C ) 2 2 1 & 2 B D = 1 3 a n d D C = 2 3 \triangle ABC\text{ is 30-60-90 } \Rightarrow BC=\sqrt { 3 } \text{ and } AC=2\\ AD\text{ is the angle bisector of } \angle BAC\\ \text{ so } \angle DAC=30º=\angle DCA\Rightarrow AD=DC\\ Now\\ BC=BD+DC=\sqrt { 3 } \rightarrow 1\\ \triangle ABD \text{ is right angle triangle} \\ { \left( AB \right) }^{ 2 }+{ \left( BD \right) }^{ 2 }={ \left( DC \right) }^{ 2 }\qquad //AD=DC\\ 1+{ \left( BD \right) }^{ 2 }={ \left( DC \right) }^{ 2 }\rightarrow 2\\ 1\& 2\Rightarrow BD=\frac { 1 }{ \sqrt { 3 } } and\quad DC=\frac { 2 }{ \sqrt { 3 } }

( the area of ) = 1 2 × r × ( the triangle’s perimeter ) \text{the area of }\triangle)=\frac{1}{2} \times r \times (\text{the triangle's perimeter})

so r A B D = 2 A r e a p e r i m e t e r = 1 3 1 + 1 3 + 2 3 = 1 3 + 3 similarly r A D C 2 A r e a p e r i m e t e r = 2 3 2 + 2 3 + 2 3 = 1 2 + 3 Area of the grey=Area of A B C (Area of the circles) A = 3 2 π ( r A D C 2 + r A D C 2 ) 0.5 \text{so }r_{ \triangle ABD }=\frac { 2Area }{ perimeter } =\frac { \frac { 1 }{ \sqrt { 3 } } }{ 1+\frac { 1 }{ \sqrt { 3 } } +\frac { 2 }{ \sqrt { 3 } } } =\frac { 1 }{ 3+\sqrt { 3 } } \\ \text{similarly } r_{ \triangle ADC }\frac { 2Area }{ perimeter } =\frac { \frac { 2 }{ \sqrt { 3 } } }{ 2+\frac { 2 }{ \sqrt { 3 } } +\frac { 2 }{ \sqrt { 3 } } } =\frac { 1 }{ 2+\sqrt { 3 } } \\ \text{Area of the grey=Area of } \triangle ABC - \text{(Area of the circles)}\\ A=\frac { \sqrt { 3 } }{ 2 } -\pi \left( r^{ 2 }_{ \triangle ADC }+r^{ 2 }_{ \triangle ADC } \right) \approx 0.5

A I
Apr 6, 2018

This question was very entertaining! The most important thing is to work out the radii of the two circles, which can be achieved by the use of trig and some simple knowledge of right angles. This lead us to the two equations: Tan(30)=r/(root 3 over 3)-r Where r is the radius of the smaller circle; And sin(60)=R/(root 3 over 3)-R Which then could be used to find the area of white bits and obtaining the shaded region as 0.5001713347.

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