blues

Geometry Level 2

The Figure shows a right triangle ABC at B (with green colour ) , and two blue lunes.The outside of each lune is a semicircle and the inside of each lune is part of the circle passing through the points A,B and C,(circumcircle of ABC). if AB = 8 and BC = 6,find the area of the two blue lunes ?

Definition of lune : A crescent-shaped figure formed on a sphere or plane by two arcs intersecting at two points.


The answer is 24.

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4 solutions

Chakravarthy B
Mar 6, 2019

the above is hippocrates theorem

the below is proof

Aziz Alasha
Jan 4, 2018

using The Pythagorean Theorem we can prove that : the area of the semicircle on AC = the sum of areas of the semicircles on AB and BC. ( By the way, this can be considered as an other extension of the Pythagorean Theorem).

Therefore ; the area of the semicircle on AC = the green area + the white area.

the sum of areas of the semicircles on AB and BC = the blue area + the white area

so far , the blue area = the green area = 24

AC is the diameter of the semi-circle formed by arcs AB and BC, given the theorem that any triangle with a base as the diameter of a circle will form a right angled triangle.

Area of the white portion = Area of the semi-circle mentioned above - Area of triangle ABC

= π \pi x ( A C / 2 ) 2 (AC/2)^{2} - (0.5 x AB x BC)

Given A C 2 AC^{2} = B C 2 BC^{2} + A B 2 AB^{2}

= π \pi x [ ( B C / 2 ) 2 + ( A B / 2 ) 2 (BC/2)^{2} + (AB/2)^{2} ] - (0.5 x AB x BC) ... (1)

Area of the semi-circles formed by the two lunes = π \pi x [ ( B C / 2 ) 2 + ( A B / 2 ) 2 (BC/2)^{2} + (AB/2)^{2} ] ... (2)

Required area of the two blue lines = (2) - (1) = (0.5 x AB x BC) = 0.5 x 6 x 8 = 24 square units

The area of the two blue lunes is equal to the area of the green triangle.

thats correct

Aziz Alasha - 3 years, 5 months ago

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Yeah! Great problem . .

A Former Brilliant Member - 3 years, 5 months ago

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