Area Of Colored Squares

Geometry Level 3

In the three colored squares above, the semicircle with diameter 5 5 passes through the vertex of the red square at point P P .

Find the total area of the three colored squares.


The answer is 14.

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7 solutions

David Vreken
Nov 2, 2020

Let the red square have a side length of x x and the purple square have a side length of y y , so that the blue square has a side length of x + y x + y , and label the diagram as follows:

Since Q R QR is a diameter, Q P R \angle QPR is a right angle, and Q S P P S R \triangle QSP \sim \triangle PSR by AA similarity.

This means that Q S P S = P S S R \cfrac{QS}{PS} = \cfrac{PS}{SR} , or y x y = y y + 2 x \cfrac{y - x}{y} = \cfrac{y}{y + 2x} , which simplifies to y = 2 x y = 2x for x 0 x \neq 0 .

Also, Q R = Q T + T R = y + ( x + y ) = x + 2 y = 5 QR = QT + TR = y + (x + y) = x + 2y = 5 .

The two equations y = 2 x y = 2x and x + 2 y = 5 x + 2y = 5 solve to x = 1 x = 1 and y = 2 y = 2 .

Therefore, the three squares have side lengths of 1 1 , 2 2 , and 1 + 2 = 3 1 + 2 = 3 , and the sum of their areas is 1 2 + 2 2 + 3 2 = 14 1^2 + 2^2 + 3^2 = \boxed{14} .

Rocco Dalto
Nov 1, 2020

h 2 = x ( 5 x ) h = x ( 5 x ) h^2 = x(5 - x) \implies h = \sqrt{x(5 - x)}

m = 5 h m h = 5 2 h m = 5 - h \implies m - h = 5 - 2h

A p i n k = h 2 A_{pink} = h^2

A l i g h t b l u e = m 2 = ( 5 h ) 2 A_{lightblue} = m^2 = (5 - h)^2

A r e d = ( m h ) 2 = ( 5 2 h ) 2 A_{red} = (m - h)^2 = (5 - 2h)^2

A w h i t e = x ( m h ) = x ( 5 2 h ) A_{white} = x(m - h) = x(5 - 2h)

5 m = 5 ( 5 h ) = h 2 + 25 10 h + h 2 + 25 20 h + 4 h 2 + 5 x 2 x h \implies 5m = 5(5 - h) = h^2 + 25 - 10h + h^2 + 25 - 20h + 4h^2 + 5x - 2xh \implies

6 h 2 + 5 x + 25 = ( 2 x + 25 ) h 6h^2 + 5x + 25 = (2x + 25)h

Using h = x ( 5 x ) h = \sqrt{x(5 - x)} \implies

6 x 2 + 35 x + 25 = ( 2 x + 25 ) x ( 5 x ) -6x^2 + 35x + 25 = (2x + 25)\sqrt{x(5 - x)}

Squaring both sides and simplifying we obtain:

8 x 4 68 x 3 + 210 x 2 275 x + 125 = 0 8x^4 - 68x^3 + 210x^2 - 275x + 125 = 0

By inspection x = 1 x = 1 is a root to the above equation and dividing 8 x 4 68 x 3 + 210 x 2 275 x + 125 8x^4 - 68x^3 + 210x^2 - 275x + 125 by x 1 x - 1 we have:

( x 1 ) ( 8 x 3 60 x 2 + 150 x 125 ) = 0 ( x 1 ) ( 2 x 5 ) 3 = 0 (x - 1)(8x^3 - 60x^2 + 150x - 125) = 0 \implies (x - 1)(2x - 5)^3 = 0

x = 1 , x = 5 2 \implies x = 1, x = \dfrac{5}{2}

x = 1 h = 2 m = 3 m h = 1 x = 1 \implies h = 2 \implies m = 3 \implies m - h = 1

x = 5 2 h = 5 2 m = 5 2 m h = 0 x = \dfrac{5}{2} \implies h = \dfrac{5}{2} \implies m = \dfrac{5}{2} \implies m - h = 0

\therefore drop x = 5 2 x = \dfrac{5}{2} and pick x = 1 x = 1

\therefore the desired area A = 2 2 + 3 2 + 1 = 14 A = 2^2 + 3^2 + 1 = \boxed{14} .

Drop P H PH perpendicular to the diameter A B AB .
Denote by x x and y y the side lengths of the light blue and red squares respectively and by O O the center of the semicircle.
Then, E L = P H = A L = 5 x EL=PH=AL=5-x . Moreover, D E + E L = D L y + ( 5 x ) = x y = 2 x 5 DE+EL=DL\Rightarrow y+\left( 5-x \right)=x\Rightarrow y=2x-5 Since, y > 0 y>0 we also have 2 x 5 > 0 x > 5 2 2x-5>0\Leftrightarrow x>\frac{5}{2} . By Pythagoras’ theorem on the right-angled O P H \triangle OPH , P H 2 + H O 2 = P O 2 ( 5 x ) 2 + ( y + x 5 2 ) 2 = ( 5 2 ) 2 ( 1 ) ( 5 x ) 2 + ( 2 x 5 + x 5 2 ) 2 = 25 4 2 x 2 11 x + 15 = 0 x = 5 2 or x = 3 \begin{aligned} P{{H}^{2}}+H{{O}^{2}}=P{{O}^{2}} & \Rightarrow {{\left( 5-x \right)}^{2}}+{{\left( y+x-\frac{5}{2} \right)}^{2}}={{\left( \frac{5}{2} \right)}^{2}} \\ & \overset{(1)}{\mathop{\Leftrightarrow }}\,{{\left( 5-x \right)}^{2}}+{{\left( 2x-5+x-\frac{5}{2} \right)}^{2}}=\frac{25}{4} \\ & \Leftrightarrow 2{{x}^{2}}-11x+15=0 \\ & \Leftrightarrow x=\frac{5}{2}\text{ or }x=3 \\ \end{aligned} Due to the restriction, the value 5 2 \frac{5}{2} is rejected. Hence x = 3 x=3 and this gives y = 1 y=1 .
Thus, the total area of the three squares is A = x 2 + y 2 + ( 5 x ) 2 = 3 2 + 1 2 + 2 2 = 14 . A={{x}^{2}}+{{y}^{2}}+{{\left( 5-x \right)}^{2}}={{3}^{2}}+{{1}^{2}}+{{2}^{2}}=\boxed{14}.


Callie Ferguson
Nov 2, 2020

We have squares of three colors: pink, red, and cyan. Let's refer to the side lengths of each square as side lengths p p (pink square), r r (red square), and c c (cyan square).

The following statements are what we know to be true about the figure, where the x x and y y values refer to the horizontal and vertical side lengths, respectively.

x p + x c = 5 x_p + x_c = 5

x r = 1 2 x p x_r = \frac{1}{2} x_p

y p + y r = y c y_p + y_r = y_c

We know, though, that the x x and y y side lengths for each respective colored square are equal. So we can now refer to, for example, x p \bold{x_p} and y p \bold{y_p} , as just p \bold{p} .

With this being true, the equations above can be written as:

(1) … p + c = 5 p + c = 5

(2) … r = 1 2 p r = \frac{1}{2} p 2 r = p \rightarrow 2r=p

(3) … p + r = c p + r = c

These are much simpler to substitute into one another to solve for variables.

\space

Solving equations (1), (2), and (3) for variables \large{\text{Solving equations (1), (2), and (3) for variables}}

Let's use (2) to solve (3) for c c :

2 r + r = c c = 3 r 2r + r = c \rightarrow c=3r

Now that we know that c = 3 r c=3r and p = 2 r p=2r , we can solve (1) in terms of r r .

2 r + 3 r = 5 2r + 3r = 5

5 r = 5 \rightarrow 5r = 5

r = 1 \rightarrow r=1

So, now that we know that r = 1 r=1 , we also know that:

From (3), where p = 2 r p=2r , 2(1) + (1) = c)

c = 3 \rightarrow c = 3

p + 3 = 5 p = 2 p + 3 = 5 \rightarrow p=2

Let the side lengths of the purple, blue, and red squares be a a , b b , and c c . We note that because Q R QR is a diameter, Q P R = 9 0 \angle QPR = 90^\circ . Then

P R 2 + P Q 2 = Q R 2 T P 2 + T Q 2 + P S 2 + S R 2 = 25 ( a c ) 2 + a 2 + a 2 + ( c + b ) 2 = 25 Note that b = 5 a and ( 3 a 5 ) 2 + a 2 + a 2 + ( 10 3 a ) 2 = 25 a + c = b c = 5 2 a 9 a 2 30 a + 25 + 2 a 2 + 9 a 2 60 a + 100 = 25 20 a 2 90 a + 100 = 0 2 a 2 9 a + 10 = 0 ( 2 a 5 ) ( a 2 ) = 0 Since a < 2.5 a = 2 \begin{aligned} \blue{PR^2} + \red{PQ^2} & = QR^2 \\ \blue{TP^2+TQ^2} + \red{PS^2+SR^2} & = 25 \\ \blue{(a-c)^2+a^2} + \red{a^2+(c+b)^2} & = 25 & \small \blue{\text{Note that }b=5-a \text{ and}} \\ (3a-5)^2 + a^2 + a^2 + (10-3a)^2 & = 25 & \small \blue{a+c = b \implies c = 5-2a} \\ 9a^2 - 30a + 25 + 2a^2 + 9a^2 - 60a + 100 & = 25 \\ 20a^2 - 90a + 100 & = 0 \\ 2a^2 - 9a + 10 & = 0 \\ (2a-5)(a-2) & = 0 & \small \blue{\text{Since }a < 2.5} \\ \implies a & = 2 \end{aligned}

Therefore b = 5 a = 3 b = 5-a = 3 and c = 5 2 a = 1 c = 5-2a = 1 and the area of the three squares is 1 2 + 2 2 + 3 2 = 14 1^2 + 2^2 + 3^2 = \boxed{14}

Vinod Kumar
Nov 14, 2020

Let red be x, purple be y and blue be z size squares. Easy to show,

z=x+y, z+y=5 and y/(y-x)=(z+y-x)/y

Therefore, x=1, y=2 and z=3, giving answer as 14.

Ron Gallagher
Nov 2, 2020

Let B = length of the side of the blue square, R = length of a side of the red square, and P = length of the side of the purple square. From the picture, we find:

B + P = 5

P + R = B

P + (P-R) = B

This gives three equations in three unknowns. Solving yields R = 1, B = 3, and P = 2. Hence, the sum of the areas is:

1^2 + 2^2 + 3^2 = 14

How did you get the third equation?

Thanos Petropoulos - 7 months, 1 week ago

Hi Thanos - Refer to the picture in Rocco Dalto's solution. The length of the side of the white square is P - R. The length of the purple square (P) plus the length of the white square (P - R) is the length of the blue square (B) - i.e., P + (P-R) = B

Ron Gallagher - 7 months, 1 week ago

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Hi Ron! I think there is still something missing: How do we know that the white rectangle is a square?

Thanos Petropoulos - 7 months, 1 week ago

Hi Thanos - You make a good point. I was largely assuming it as "obvious from the picture", which (of course) can lead to faulty conclusions. However, we can prove it as follows. By the way, I realize I called the length of the purple square "P", which was used as a point in the problem. In what follows, P is the length of the purple square.

Choose a coordinate system with the center of the semicircle at the origin. The radius of the circle is given as 2.5. Let x be defined as the distance between the origin and the lower right hand corner of the purple square. Then, recognizing that the points that points (B-x,0), (-x-P, 0), and (-x-R, P) all lie on the circle, the following equations must be satisfied:

P + x + B = 5

(x+R)^2 + P^2 = 2.5^2

(B-x)^2 = 2.5^2

(-x-P)^2 = 2.5^2

Solving these equations gives the desired result.

Ron Gallagher - 7 months, 1 week ago

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Ok now. I don't think one could avoid using Pythagoras' theorem in a way or an other:)

Thanos Petropoulos - 7 months, 1 week ago

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I did by using similar triangles instead :-)

David Vreken - 7 months, 1 week ago

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@David Vreken Well done David! Anyway, I was wrong, since Pythagoras' theorem can be proved by the similarity of P Q R \triangle PQR , R P S \triangle RPS and R Q P \triangle RQP (as labeled in your figure).

Thanos Petropoulos - 7 months, 1 week ago

Sorry, the first equation should read x + P = 2.5, not x+P + B = 5. Not sure how to edit a comment...

Ron Gallagher - 7 months, 1 week ago

I think you’re right. In response to your question “How did I know it is a square?”, the answer is “I got lucky”. Better to be lucky than good sometimes. Thanks for your input

Ron Gallagher - 7 months, 1 week ago

You wellcome!

Thanos Petropoulos - 7 months, 1 week ago

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