I want to draw a Moon

Geometry Level 3

What is the area of the large square?


The answer is 256.

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18 solutions

I've labeled a few points of interest in the figure, which will be showcased here:

Since B C \overline{BC} is the diameter of the small circumference, and A is a point within the circumference itself, then A B C \overline{ABC} is a right triangle. Furthermore, A H \overline{AH} is perpendicular to B C \overline{BC} , thus A H \overline{AH} is the altitude of the triangle with respect to the hypotenuse.

Let R R be the radius of the large circumference. Thus, it is obvious that the following segments measure, respectively:

A H = R 4 \overline{AH} = R - 4

B H = R 6 \overline{BH} = R - 6

C H = R \overline{CH} = R

From the right angle altitude theorem, we can write: A H 2 = B H C H \overline{AH}^{2} = \overline{BH}*\overline{CH}

Then:

( R 4 ) 2 = R ( R 6 ) (R - 4)^{2} = R*(R - 6)

R 2 8 R + 16 = R 2 6 R R^{2} - 8R + 16 = R^{2} - 6R

2 R = 16 2R = 16

R = 8 R = 8

Since the side of the square is equal to the diameter of the large circle, then its side is 16 16 ; thus, the area equals 1 6 2 = 256 16^{2} = 256

this problem is in math lesson of nine grade in vietnam :v

Vũ An - 6 years, 4 months ago

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Yeah. It's a 9th standard problem in India too. :3

Ananya Prakash - 5 years, 6 months ago

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Really I was not taught altitude theorem then

Ashish Yadav - 3 years, 6 months ago

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@Ashish Yadav This is not exactly a theorem. Infact, in Nepal it is considered as a solving method where you have to show the full process using similarity between triangles.

इश्वर बस्याल - 3 years, 6 months ago

Ask them to do it again in 50 yrs time! I am attempting these to keep my brain active. 63 yr old.

Kev Barry - 3 years, 9 months ago

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Respectful!

Vũ An - 3 years ago

I didn't think i can encounter a genius man like you! Thumbs up! I stared at the problem for20 minutes and cannot devise a solution like yours. Thanks.

Cabanting Perez Francis - 6 years, 4 months ago

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nothing grest bro....all we did'nt remember is the "theorem stuff"......isn't it?????

Manikanta Yerraguntla - 6 years, 4 months ago

right angle altitude theorem, we can write AH2=BH*CH........???

Manikanta Yerraguntla - 6 years, 4 months ago

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Not really a "theorem". You don't need to memorize it either. Triangle AHC and BHA are similar. He just equated sinA in AHC and sinB in BHA.

Ananya Prakash - 5 years, 6 months ago

This is not explained in details in his solution, but if "O" will be point marking the middle of the circle, then angle ∠BAC, which touches the circle (and is constructed on an arc BC), have half the degrees of the angle constructed on the same arc, but coming from the middle of the circle, that is ∠BOC. Because ∠BOC have 180 degrees, then ∠BAC have 90 degrees, i.e. it's a right angle.

Now, all you need for |AH|^2 = |BH|*|HC| is Pythagorean theorem, used three times. You have that

|AB|^2 + |AC|^2 = (|BH|+|HC|)^2 = |BH|^2 +2|BH||HC| + |HC|^2

Because

|BH|^2 = |AB|^2 - |AH|^2

|HC|^2 = |AC|^2 - |AH|^2

then putting it into previous equality, we get

0 = 2|BH||HC| - 2 |AH|^2

|AH|^2 = |BH||HC|

Scarlet Evans - 3 years, 3 months ago

Hi, is the right angle theorem a scalar product or a simple multiplication between vector lengths??

Vatea Tahiti - 4 years, 9 months ago

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no vector dear, he just equated showed ABH and AHC triangles similar and equated the ratios of the sides.

इश्वर बस्याल - 3 years, 6 months ago

Nice solution. (y) ... though one particular point led me to think, that the solution is purely based on a particular assumption. The lines having segments 6 units and 4 units are diameters of the large circle indeed, but, it was never said in the problem that those two diameters are perpendicular to each other...we assume that, and solve the problem. @Banti Paswan please , have a look.

Suman Roy - 4 years, 3 months ago

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Well, you're right to state I did not prove that the diameters are perpendicular. I actually don't think it matters, but I can give a brief explanation as to why that is.

The upper side of the square is, by definition, tangent to the circle (if it isn't it renders our problem pretty much unsolvable). Therefore, the radius drawn from the center of the circle to the tangent point creates a right angle there. But since the upper right corner of the square also forms a right angle, these lines drawn are parallel.

A similar argument can be made to prove that the diameter that crosses the left and right sides of the square also forms a 90 degree angle at the tangent point, which leaves us with no option but to conclude that the last remaining angle in the quadrilateral formed by one of the square's corners, two non-opposing tangent points and the circle's center is 90 degrees. Since all angles are 90 degrees, and two non-opposing sides have the same length, we have a square, and we can safely proceed with the fact that the angle is indeed 90 degrees.

Alexandre Miquilino - 4 years, 2 months ago

visually, the red line of 4 is NOT half of the distance to H, so 16 cannot be correct as that is drawn

Richard Schnure - 2 years, 11 months ago

IF line 4 is equal to AH, then that may be the correct solution..as drawn, it clearly is not

Richard Schnure - 2 years, 11 months ago

The very 1st assumption that ABC is a right triangle . Is invalid

Don Cartmill - 2 years, 5 months ago

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Sorry. ABC would be inscribed in a semicircle with the diameter as the hypotenuse. ABC absolutely IS a right triangle.

Bill Weihmiller - 2 years, 4 months ago

You can really just use estimation that if the line on the left is 3/4 if he way to the middle and the top line is 1/2 that it would naturally be 8 from the top/sides to the middle so then you double that to get 16 and square it

Caroline Co - 2 years, 5 months ago

I had never heard of this right angle theorum until now. So any time you take two points at opposite ends of the diameter line of a circle, any third point on the circumference will form a right angle with them? Is there a specific name of this theorum so I can look it up?

Joel Shircliff - 1 year, 10 months ago

I don’t think this is correct either

Lucy Johnson - 5 months, 1 week ago

This is all theoretical maths. The actual diagram is incorrect as the 4 should be 3.8 and the area of the large square is 324. I've drawn it in AutoCAD and the figures shown are impossible.

Kevin Hough - 4 years, 9 months ago

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I don't know how you drew it in Autocad. There isn't enough information to draw the figure without solving it. Rather than repeat the proof, consider, from hindsight, the location of the center of the small circle, P. From the proof, it is 5 units away from the point C, and 5 units away from the point B. Since the diameter of the large circle is 16, that means point H is 3 units away from the point P. We require that the point A be on the small circle, so it must be 5 units away from the center, P. Then, it is clear that triangle PHA is a 3-4-5 right triangle. Thus, the point A is 4 units from both the center and the edge. This confirms the legitimacy of the math. I will agree that, as is often the case, the drawing is not to scale. But the measurements specified are all consistent. If that's not enough, triangles HBA, HAC, and ABC are all similar, and BH:AH::AH:CH::BA:CA, or 2:4::4:8::2root5:4root5. BH = 2, AH = 4, and CH = 8.

Tom Capizzi - 4 years, 1 month ago

Drawing it in autocad is also theoretical math. In order for you to say 4 is incorrect you need to say the 6 alone (along with the rough shape) is enough to determine the lengths. But it isn't. Instead you need to assume that the distance measured from the centre of the larger circle/square is the same to the centre of the smaller circle as it is to the edge of the smaller circle, i.e. that the radius of the smaller circle is 6.

But no where is that indicated.

(unless you are trying to suggest the shape was drawn perfectly above so you can just measure it with a ruler, but that isn't what you have).

Mr Noone - 3 years, 2 months ago

can you wxplain this process in a easier way please....????/

Pranab Kalita - 6 years, 4 months ago

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This is the easiest.What are you talking about?

इश्वर बस्याल - 3 years, 6 months ago
Rahul Paswan
Jan 21, 2015

AO = y-4 = (x+3) -4 = x-1, use Pythagoras (x-1)^2 + 3^2 = x^2 , x^2 -2x +1 +9= x^2, 2x=10, x=5 , y=5+3=8. 16^2=256 Just neatens up your solution method!

Charles Loft - 6 years, 4 months ago

Unique and nice way

Harsh Malra - 4 years, 4 months ago

Nice solution. (y) ... though one particular point led me to think, that the solution is purely based on a particular assumption. The lines having segments 6 units and 4 units are diameters of the large circle indeed, but, it was never said in the problem that those two diameters are perpendicular to each other...we assume that, and solve the problem. @Banti Paswan , please , have a look.

Suman Roy - 4 years, 3 months ago

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The diameters are perpendicular to adjacent sides of a square, so they must be perpendicular to each other

Chris Maitland - 2 years, 10 months ago

FAB.........

Manikanta Yerraguntla - 6 years, 4 months ago
Paola Ramírez
Jan 21, 2015

Let x x radii of the biggest circunference.

We have that C B = x 6 CB=x-6 and A B = x 4 AB=x-4 . Now A C D \triangle ACD and A B AB is its heigth so A B C D B A \triangle ABC \sim \triangle DBA .

C B A B = A B B D \frac{CB}{AB}=\frac{AB}{BD}

x 6 x 4 = x 4 x \frac{x-6}{x-4}=\frac{x-4}{x}

x 2 6 x = x 2 8 x + 16 x^2-6x=x^2-8x+16

2 x = 16 2x=16

x = 8 x=8

Side of the square is equal to 2 x 2x \therefore side longitude is 16 16 \Rightarrow area is 256 \boxed{256}

Nicely done.

Peter Byers - 4 years, 10 months ago

m(CAD)=90 Öklid teoremi

Soner Karaca - 4 years, 9 months ago

Very elegant!

Matthias Semlinger - 4 years, 7 months ago

The relationship of the triangles is a neat method. Thanks.

Kev Barry - 3 years, 9 months ago

This is the best and easiest to understand. Thanks

Karim Jivani - 3 years, 5 months ago

For circumference I think you meant circle.

Bill Weihmiller - 2 years, 4 months ago

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Yes, I meant a circle

Paola Ramírez - 2 years, 2 months ago

Handwriting is somehow much easier to read than printed text. Very clear to follow.

Denise Chevalier - 3 years, 10 months ago
Gamal Sultan
Jan 24, 2015

Let the center of the large circle be O and its radius be r

Let the contact point of the two circles be A

The horizontal diameter of the large circle cuts the small circle at C

The vertical diameter of the large circle cuts the small circle at B above the

horizontal diameter

In the small circle

AC is a diameter

Then

angle ABC is right

In the right triangle ABC

Since BO is perpendicular to AC

Then

(BO)^2 =OA X OC

(r - 4)^2 = r (r - 6)

r = 8

The area of the large square = 16^2 = 256

Anna Anant
Jan 24, 2015

Let x be the radius of the large circle. Then connect the three points shown to make a triangle. By using the Pythagorean theorem we have the following equality. [(x-6)² + (x-4)²] + [(x-4)² + x²] = [(x-6)+x]² => x = 8 so area of large square is (2x)² = 16² = 256 sq units

r + y = 2 r + 6 2 r+y=\dfrac{2r+6}{2}

2 r + 2 y = 2 r + 6 2r+2y=2r+6

2 y = 6 2y=6

y + 3 y+3

2 r + 6 = 8 + 2 x 2r+6=8+2x

2 r + 2 + 2 x 2r+2+2x

r = 1 + x r=1+x ( 1 ) \color{#D61F06}(1)

By the pythagorean theorem, we have

r 2 = x 2 + y 2 r^2=x^2+y^2 ( 2 ) \color{#D61F06}(2)

\implies Substitute y=3 and equation ( 1 ) \text{Substitute y=3 and equation} \color{#D61F06}(1) in ( 2 ) \color{#D61F06}(2)

( 1 + x ) 2 = x 2 + 3 2 (1+x)^2=x^2+3^2

1 + 2 x + x 2 = x 2 + 9 1+2x+x^2=x^2+9

1 + 2 x = 9 1+2x=9

2 x = 8 2x=8

x = 4 x=4

Therefore the area of the big square is four times the area of the small square, we have

A = 4 ( 4 + x ) 2 = 4 ( 4 + 4 ) 2 = 4 ( 64 ) = 256 A=4(4+x)^2=4(4+4)^2=4(64)=256

This is my own solution. This is my deactivated account.

A Former Brilliant Member - 1 year, 7 months ago
John Miller
Jan 17, 2017

The diagram on the left shows the problem as stated. The large circle is irrelevant and was omitted. Point O is at the center of the square. Point A is where the circle intersects the y-axis above and point B is where the circle intersects the x-axis to the left. The diagram on the right shows the circle re-positioned such that it is now centered on point O. Points C and D represent the upper and lower intersections between the circle and the y-axis. Line CD also represents the diameter of the circle. Points A' and B' represent the same points on the circle which now lie in new locations within the square. Triangle CAA' is a right triangle with legs of CA=1 and AA'=3. Since line CD is the diameter of the circle, angle CA'D must be a right angle. Thus, by the Pythagorean theorem, DA * AC = 3^2, so DA * 1 = 9, thus DA = 9. Therefore the diameter of the circle (line CD) is 10. Thus, the length of a side of the square is 16 and the area of the square is 256. In my initial solution, I defined a triangle with vertexes at C, O, and the midpoint of CA'. Then I used trig to find the radius (CO) of the circle. As I was writing that up, I noticed I could use Pythagoras instead.

That's a nice translation trick!

Calvin Lin Staff - 4 years, 4 months ago
Fadi BouKaram
Aug 31, 2019

I didn't include the larger circle in the image because it is irrelevant. Let the square be of side: 2 a 2a , with its surface area being 4 a 2 4a^2 .

The radius of the circle is: 2 a 6 2 = a 3 O B = O A = a 3 \frac{2a-6}{2} = \scriptstyle a-3 \Rightarrow OB = OA = a-3 .

O C = O B B C = ( a 3 ) ( a 6 ) = 3 \scriptstyle OC= OB - BC = (a-3) - (a-6) = 3

O A 2 = A C 2 + O C 2 ( a 3 ) 2 = ( a 4 ) 2 + 9 \scriptstyle OA^2= AC^2 + OC^2 \space \Rightarrow \space (a-3)^2 = (a-4)^2 + 9

a 2 6 a + 4 = a 2 8 a + 16 + 9 a = 8 A = 256 \scriptstyle a^2 -6a + 4 = a^2 - 8a + 16 + 9 \space \Rightarrow \space a = 8 \space \Rightarrow \space A = 256

Anagha Varma
Jul 30, 2018

By looking at the diagram, you can see that the line of length 4 divides the height of the small square into 2. So, the height of the side of the small square is 8. So the area of the small square is 64. As there are 4 such squares, the total area is 256. It really is a lot about intuition. Not a very recommended method but fast.

Megan DeSerres
Dec 2, 2015

Since the line of 4 is half of the smaller square therefore 8 is the total height and base if the smaller square. 8 * 8= 64 and since there are 4 squares 64 * 4= 256

Jayaram Dorai
Jun 16, 2015

Using notation as below and assuming r as the radius of the small circle: Diameter of large circle = CB + 8 = 2r + 6. So radius of large circle = CH = r + 3. BH = (r + 3) - 6 = r - 3. AH = HD = (r + 3) - 4 = r - 1 (assuming D is the intersection of AH produced on the small circle). But AH HD = BH HC. So (r + 3) (r - 3) = (r - 1) (r - 1) Solving for r, we have r = 5. So diameter of the large circle = 2r + 6 = 16 and area of the large square = 256.

Tom Schulte
Jan 27, 2015

The diameter of the large circle is 2s where s is the unknown side half-length of the large square. This makes the diameter of the small circle 2s -6 and then the radius s - 3 and if the central lines cross at the origin of a Cartesian coordinate system, the small circle has two x-intercepts: (s, 0) and (-s + 6, 0) and thus has center ( (s + -s + 6)/2, 0 ) or (3,0)

This makes the equation of the smaller circle

(x - 3)^2 + y^2 = (s -3)^2

We know some points on the circle, like the uppermost y-intercept (0, s-4)

Plug that in for x = 0 and y = 4 and solve for s = 8

The square has sides 2*s = 16 and area s^2 = 256.

Nice puzzle!

Gabriel Chacón
Jan 22, 2019

A B C ABC is a right triangle . Therefore A B 2 + B C 2 = A C 2 \overline{AB}^2+\overline{BC}^2=\overline{AC}^2 .

Express the Pythagorean identity in terms of x , y x,y and r r : ( x 2 + y 2 ) + ( y 2 + r 2 ) = ( x + r ) 2 (x^2+y^2)+(y^2+r^2)=(x+r)^2 .

Simplify to get y 2 = x r y^2=x\cdot r

Substitute x = r 6 x=r-6 and y = r 4 y=r-4 to get ( r 4 ) 2 = r ( r 6 ) r 2 + 16 8 r = r 2 6 r r = 8 (r-4)^2=r(r-6) \implies r^2+16-8r=r^2-6r \implies r=8 .

The area of the large square is then 4 r 2 = 256 4r^2=\boxed{256} .

I’m probably the only person who solved it with pixel measurements lol

6 + A B = 4 + B D 6+AB=4+BD \implies B D = 2 + A B BD=2+AB or A B = B D 2 AB=BD-2

B C = 2 F D + 6 2 = F D + 3 BC=\dfrac{2FD+6}{2}=FD+3

F D + 3 = 4 + B D FD+3=4+BD \implies F D = B D + 1 FD=BD+1

B F = F D A B = B D + 1 ( B D 2 ) = B D + 1 B D + 2 = 3 BF=FD-AB=BD+1-(BD-2)=BD+1-BD+2=3

Using pythagorean theorem, we have

F D 2 = B D 2 + B F 2 FD^2=BD^2+BF^2

( B D + 1 ) 2 = B D 2 + 9 (BD+1)^2=BD^2+9

B D 2 + 2 B D + 1 = B D 2 + 9 BD^2+2BD+1=BD^2+9

2 B D = 8 2BD=8

B D = 4 BD=4

So the radius of the big circle is 4 + 4 = 8 4+4=8 .

Thus, the area of the square is 1 6 2 = 16^2= 256 \boxed{256} .

Jon Danford
Jan 22, 2017

The figure looks to scale. So what I did was look at the smaller inside squares instead of the circles. It looks like they have side lengths of 8, making the bigger squares side lengths 16. A = l X w. 16 X 16 = 256.

Conor Collins
Jan 26, 2016

If you compare 4 and 6 you end up 8 as your small square size double to 16 multiply by 2 and there's your answer of 256 without all the head racking

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