In the three colored squares above, the semicircle with diameter 5 passes through the vertex of the red square at point P .
Find the total area of the three colored squares.
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h 2 = x ( 5 − x ) ⟹ h = x ( 5 − x )
m = 5 − h ⟹ m − h = 5 − 2 h
A p i n k = h 2
A l i g h t b l u e = m 2 = ( 5 − h ) 2
A r e d = ( m − h ) 2 = ( 5 − 2 h ) 2
A w h i t e = x ( m − h ) = x ( 5 − 2 h )
⟹ 5 m = 5 ( 5 − h ) = h 2 + 2 5 − 1 0 h + h 2 + 2 5 − 2 0 h + 4 h 2 + 5 x − 2 x h ⟹
6 h 2 + 5 x + 2 5 = ( 2 x + 2 5 ) h
Using h = x ( 5 − x ) ⟹
− 6 x 2 + 3 5 x + 2 5 = ( 2 x + 2 5 ) x ( 5 − x )
Squaring both sides and simplifying we obtain:
8 x 4 − 6 8 x 3 + 2 1 0 x 2 − 2 7 5 x + 1 2 5 = 0
By inspection x = 1 is a root to the above equation and dividing 8 x 4 − 6 8 x 3 + 2 1 0 x 2 − 2 7 5 x + 1 2 5 by x − 1 we have:
( x − 1 ) ( 8 x 3 − 6 0 x 2 + 1 5 0 x − 1 2 5 ) = 0 ⟹ ( x − 1 ) ( 2 x − 5 ) 3 = 0
⟹ x = 1 , x = 2 5
x = 1 ⟹ h = 2 ⟹ m = 3 ⟹ m − h = 1
x = 2 5 ⟹ h = 2 5 ⟹ m = 2 5 ⟹ m − h = 0
∴ drop x = 2 5 and pick x = 1
∴ the desired area A = 2 2 + 3 2 + 1 = 1 4 .
P
H
perpendicular to the diameter
A
B
.
Denote by
x
and
y
the side lengths of the light blue and red squares respectively and by
O
the center of the semicircle.
Then,
E
L
=
P
H
=
A
L
=
5
−
x
.
Moreover,
D
E
+
E
L
=
D
L
⇒
y
+
(
5
−
x
)
=
x
⇒
y
=
2
x
−
5
Since,
y
>
0
we also have
2
x
−
5
>
0
⇔
x
>
2
5
.
By Pythagoras’ theorem on the right-angled
△
O
P
H
,
P
H
2
+
H
O
2
=
P
O
2
⇒
(
5
−
x
)
2
+
(
y
+
x
−
2
5
)
2
=
(
2
5
)
2
⇔
(
1
)
(
5
−
x
)
2
+
(
2
x
−
5
+
x
−
2
5
)
2
=
4
2
5
⇔
2
x
2
−
1
1
x
+
1
5
=
0
⇔
x
=
2
5
or
x
=
3
Due to the restriction, the value
2
5
is rejected. Hence
x
=
3
and this gives
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
=
1
4
.
We have squares of three colors: pink, red, and cyan. Let's refer to the side lengths of each square as side lengths p (pink square), r (red square), and c (cyan square).
The following statements are what we know to be true about the figure, where the x and y values refer to the horizontal and vertical side lengths, respectively.
x p + x c = 5
x r = 2 1 x p
y p + y r = y c
We know, though, that the x and y side lengths for each respective colored square are equal. So we can now refer to, for example, x p and y p , as just p .
With this being true, the equations above can be written as:
(1) … p + c = 5
(2) … r = 2 1 p → 2 r = p
(3) … p + r = c
These are much simpler to substitute into one another to solve for variables.
Solving equations (1), (2), and (3) for variables
Let's use (2) to solve (3) for c :
2 r + r = c → c = 3 r
Now that we know that c = 3 r and p = 2 r , we can solve (1) in terms of r .
2 r + 3 r = 5
→ 5 r = 5
→ r = 1
So, now that we know that r = 1 , we also know that:
From (3), where p = 2 r , 2(1) + (1) = c)
→ c = 3
p + 3 = 5 → p = 2
Let the side lengths of the purple, blue, and red squares be a , b , and c . We note that because Q R is a diameter, ∠ Q P R = 9 0 ∘ . Then
P R 2 + P Q 2 T P 2 + T Q 2 + P S 2 + S R 2 ( a − c ) 2 + a 2 + a 2 + ( c + b ) 2 ( 3 a − 5 ) 2 + a 2 + a 2 + ( 1 0 − 3 a ) 2 9 a 2 − 3 0 a + 2 5 + 2 a 2 + 9 a 2 − 6 0 a + 1 0 0 2 0 a 2 − 9 0 a + 1 0 0 2 a 2 − 9 a + 1 0 ( 2 a − 5 ) ( a − 2 ) ⟹ a = Q R 2 = 2 5 = 2 5 = 2 5 = 2 5 = 0 = 0 = 0 = 2 Note that b = 5 − a and a + c = b ⟹ c = 5 − 2 a Since a < 2 . 5
Therefore b = 5 − a = 3 and c = 5 − 2 a = 1 and the area of the three squares is 1 2 + 2 2 + 3 2 = 1 4
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.
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?
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
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Hi Ron! I think there is still something missing: How do we know that the white rectangle is a square?
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.
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Ok now. I don't think one could avoid using Pythagoras' theorem in a way or an other:)
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I did by using similar triangles instead :-)
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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 , △ R P S and △ R Q P (as labeled in your figure).
Sorry, the first equation should read x + P = 2.5, not x+P + B = 5. Not sure how to edit a comment...
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
You wellcome!
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Let the red square have a side length of x and the purple square have a side length of y , so that the blue square has a side length of x + y , and label the diagram as follows:
Since Q R is a diameter, ∠ Q P R is a right angle, and △ Q S P ∼ △ P S R by AA similarity.
This means that P S Q S = S R P S , or y y − x = y + 2 x y , which simplifies to y = 2 x for x = 0 .
Also, Q R = Q T + T R = y + ( x + y ) = x + 2 y = 5 .
The two equations y = 2 x and x + 2 y = 5 solve to x = 1 and y = 2 .
Therefore, the three squares have side lengths of 1 , 2 , and 1 + 2 = 3 , and the sum of their areas is 1 2 + 2 2 + 3 2 = 1 4 .