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Geometry Level 2

An equilateral triangle is circumscribed, find the area of the triangle

Give your answer to 3 decimal places.


The answer is 5.1961524227066319.

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3 solutions

Chew-Seong Cheong
Dec 14, 2014

Let the triangle be A B C \triangle ABC , A A being at the top, B C BC the base, and the center of the unit radius incircle be O O . Then A O AO , B O BO and C O CO divide internally the A \angle A , B \angle B and C \angle C respectively. Let the incircle touches B C BC at M M . Due to symmetry, M M is the midpoint of B C BC .

The area of the triangle = 1 2 × B C × A M = \dfrac {1} {2} \times BC \times AM

= 1 2 ( 2 × B M ) ( A O + O M ) \quad \quad \quad \quad \quad \quad \quad \quad \quad = \dfrac {1}{2} \left( 2\times BM \right) \left( AO+OM \right)

= 1 2 ( 2 × 1 tan 3 0 ) ( 1 + 1 sin 3 0 ) \quad \quad \quad \quad \quad \quad \quad \quad \quad = \dfrac {1}{2} \left( 2 \times \dfrac {1} {\tan {30^\circ} } \right) \left( 1+\dfrac {1}{\sin{30^\circ} } \right)

= 1 2 ( 2 1 3 ) ( 1 + 1 1 2 ) \quad \quad \quad \quad \quad \quad \quad \quad \quad = \dfrac {1}{2} \left( \dfrac {2} {\frac {1}{\sqrt{3}} } \right) \left( 1+\dfrac {1}{\frac {1}{2} } \right)

= 1 2 ( 2 3 ) ( 1 + 2 ) = 3 3 5.196 \quad \quad \quad \quad \quad \quad \quad \quad \quad = \dfrac {1}{2} \left( 2\sqrt{3} \right) \left( 1+2 \right) = 3\sqrt{3} \approx \boxed {5.196}

How did you get BC and AM?

Ritu Roy - 6 years, 6 months ago

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Ritu, sorry, I didn't explain it clearly. Sorry also B C = 2 tan 3 0 BC = \dfrac {2}{\tan {30^\circ} } . Thanks for your questions, I have edited the solution.

We note that O B C = 3 0 \angle OBC = 30^\circ . Let the radius of the incircle be r = 1 r=1 . Then r B M = tan 3 0 B M = r tan 3 0 \dfrac {r} {BM} = \tan {30^\circ} \Rightarrow BM = \dfrac {r}{\tan {30^\circ} } and B C = 2 B M = 2 × r tan 3 0 = 2 × 1 1 3 = 2 3 BC = 2 BM = 2 \times \dfrac {r}{\tan {30^\circ} } = 2 \times \dfrac {1}{\frac {1}{\sqrt{3}}} = 2\sqrt{3} .

Again, O A B = 3 0 \angle OAB = 30^\circ and A M = A O + O M AM = AO + OM , O M = r = 1 OM = r = 1 and r A O = sin 3 0 A O = r sin 3 0 = 1 sin 3 0 \dfrac {r}{AO} = \sin {30^\circ} \Rightarrow AO = \dfrac {r}{\sin {30^\circ} } = \dfrac {1}{\sin {30^\circ} } . Therefore, A M = 1 + 1 sin 3 0 AM = 1 + \dfrac {1}{\sin {30^\circ} } .

Chew-Seong Cheong - 6 years, 6 months ago

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Thanks! Now I get it!!

Ritu Roy - 6 years, 5 months ago

You can divide the triangle into 6 congruent 30-60-90 triangles. 1 being the side opposite to the 30 degree angle. Hence, the base of the smaller triangle will be sqrt(3). Using this, we can find the area of the smaller triangle, [1xsqrt(3)]/2=0.866~. We multiply the area by 6, since we divided the triangle into 6 congruent parts, getting the answer 6x0.866=5.196.

Crank Tanvir
Jan 9, 2015

If we add the top of the triangle with the center of the circle we will get another triangle there.In this case we know that the hypotenuse is twice the radius . The radius of the circle is 1 so, the hypotenuse is 2 × 1 2\times 1 now use the theorem of the pithagoras x 2 + 1 2 = 2 2 x^2+1^2=2^2 ;[Base=x] so, x= 3 \sqrt{3} now, the length of equilateral triangle is 2 × x 2 \times x so, 2 × 3 2\times\sqrt{3} =3.46) here a=3.46 ;[a is the length of the equilateral triangle] now, we know the formula of the area of equilateral triangle

by calculating we will get the area of the equilateral triangle so, the answer is 5.2

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