Inscribed and Circumscribed Circles

Geometry Level 2

A B C D ABCD is a square. Γ 1 \Gamma_1 is a circle that circumscribes A B C D ABCD (i.e. Γ 1 \Gamma_1 passes through points A , B , C A, B, C and D D ). Γ 2 \Gamma_2 is a circle that is inscribed in A B C D ABCD (i.e. Γ 2 \Gamma_2 is tangential to sides A B , B C , C D AB, BC, CD and D A DA ). If the area of Γ 1 \Gamma_1 is 100 100 , what is the area of Γ 2 \Gamma_2 ?


The answer is 50.

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10 solutions

Fildah Amalia
May 20, 2014

Suppose the length of each side of A B C D ABCD is 2 s 2s

By drawing a line from center point to point A A and a line from center point perpendicularly to A B AB , we have a right triangle and get the radius of Γ 1 Γ_1 is s 2 s\sqrt{2} and Γ 2 Γ_2 is s s

Here we have

π ( s 2 ) 2 = 100 \pi (s\sqrt{2})^2 = 100

2 π s 2 = 100 2\pi s^2 = 100

and get

π s 2 = 50 \pi s^2 = 50 which is the area of Γ 2 Γ_2

Since circles are similar to each other, the ratio of their areas is the square of the ratio of their radii.

Calvin Lin Staff - 7 years ago
Aman Tiwari
May 20, 2014

Let the side of the square be a. Radius of the circumscribed circle =half the diagonal of the square=√2a/2. Radius of the inscribed circle=half the side of the square=a/2. Area of the circumscribed circle=2a^2/4 *π=100 ⇒a^2/4=50/π. Area of the inscribed circle=a^2/4 * π=50/π * π=50

Udbhav Singh
May 20, 2014

Both circle have their centers on the center of the square. Thus the radius of the inscribed circle is equal to the side length of the square ABCD (call it a a ). The circumscribing circle has a radius equal to a 2 a\sqrt{2} . Thus the ratio of the areas of the circumscribing circle to that of the inscribed circle = 1 2 \frac{1}{2} . The area of the inscribed circle is therefore = 1 2 × 100 = 50 = \frac{1}{2} \times 100 = 50

Tran Trung Nguyen
May 20, 2014
  • AC cut BD at O. => O is the center of T1 and also the center of T2.
  • The area is OA^2 x pi = 100, so OA=R1=square root of (100/pi).
  • As ABO is an isosceles and right-angled at O, according to Pytago theorem, we have OA^2 + OB^2 = AB^2
    => (100/pi) +(100/pi) = AB^2 => AB^2 = (200/pi).
  • Let H be the foot of the perpendicular from O to BC, the triangle OBC is a right-angled triangle at H. And BH^2 = (BC^2)/4 = (AB^2)/4 = (50/pi) => OB^2 - BH^2 = OH^2 => (100/pi) - (50/pi) = OH^2 => OH^2 = 50/pi
  • We have OH is the radius of T2 - the circle inscribed in ABCD, so the area of T2 is OH^2 x pi = (50/pi) x pi = 50.
Kushagra Agrawal
May 20, 2014

Let side of the square be a. The circumscribed circle Γ1 has an area of 100 and clearly it's radius is half of the diagonal of the square i.e. x=a/2^0.5

Therefore 100= pi * x * x (i)

Let area of the inscribed circle be y. Clearly the radius of this circle is half the side of the square.

z=a/2

y= pi * z * z (ii)

Dividing (i) and (ii)

we get y=50 sq. unit

Obwj Obid
May 20, 2014

let A B C D ABCD has side length of x x , Then Γ 1 Γ_{1} will have area π ( 2 x 2 ) 2 \pi(\frac{\sqrt{2}\ x}{2})^{2} = = π x 2 2 \frac{\pi\ x^{2}}{2} ,and Γ 2 Γ_{2} will have area π ( x 2 ) 2 \pi(\frac{x}{2})^{2} = = π x 2 4 \frac{\pi\ x^{2}}{4} .Now, Γ 1 Γ 2 \frac{Γ_{1}}{ Γ_{2}} = = 2 2 . Thus the area of Γ 2 Γ_{2} is 100 2 \frac{100}{2} = = 50 50

Yogesh Shivani
May 20, 2014

Let the side of the square be a units. Radius of outer circle = a/(sqrt. 2) Equating to area = 100, we get pi*(a^2) = 200

Radius of inner circle = a/2 Area = pi (a^2)/4 Substituting the value of pi (a^2); we get area = 50 units

Timothy Kurniawan
May 20, 2014

Γ 1 \Gamma_1 and Γ 2 \Gamma_2 have the same centre. assume it's O O and the radius Γ 2 \Gamma_2 is r 2 r_2 , radius of Γ 1 \Gamma_1 is r 1 r_1 . The radius of the Γ 1 \Gamma_1 is a half of square ABCD \text{ABCD} sides.The radius of Γ 2 \Gamma_2 is a half of square ABCD \text{ABCD} diagonal. then r 2 = 2 r 1 2 r 2 2 = r 1 2 2 r_2=\frac{\sqrt{2}r_1}{2}\Leftrightarrow r_2^2=\frac{r_1^2}{2} . We know area of Γ 1 \Gamma_1 is π r 1 2 = 100 \pi r_1^2=100 . Hence, area of Γ 2 \Gamma_2 is π r 2 2 = π r 1 2 2 = 100 2 = 50 \pi r_2^2=\frac{\pi r_1^2}{2}=\frac{100}{2}=50 . *p.s. sorry my english realy bad

Labelled wrongly.

Calvin Lin Staff - 7 years ago
Calvin Lin Staff
May 13, 2014

Let O O be the center of the circles, and E E be the perpendicular from O O to A B AB . Γ 1 \Gamma_1 and Γ 2 \Gamma_2 are similar by a ratio of O A O E = 2 \frac {OA}{OE} = \sqrt{2} . Hence, the area of Γ 2 \Gamma_2 is 100 ( 2 ) 2 = 50 \frac {100}{\left(\sqrt{2}\right) ^2} = 50 .

Hadia Qadir
Sep 7, 2015

Let side of the square be a. The circumscribed circle Γ1 has an area of 100 and clearly it's radius is half of the diagonal of the square i.e. x=a/2^0.5 Therefore 100= pi * x * x (i) Let area of the inscribed circle be y. Clearly the radius of this circle is half the side of the square. z=a/2 y= pi * z * z (ii) Dividing (i) and (ii) we get y=50 sq. un

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