The diagram shows an equilateral triangle with side 1. This problem was made with the help of Eléa my favourite student.
The pattern of inscribed circles (top, left and right) is repeated infinitely. The incircle of the big triangle is excluded from this pattern. The area of those circles is noted :
β
In the central part of the largest triangle we see that its incircle circumscribes an hexagon. This hexagon circumscribes a smaller hexagon; each vertex of the smaller hexagon is the middle of each side of the bigger hexagon. This pattern is repeated infinitely. The non-overlapping area of those hexagons forms an infinite number of isosceles triangles. We use one row of isosceles triangles to inscribe a black square each time. We name the total area of the infinite black squares :
α
We use another row of isosceles triangle to inscribe a black circle each time, infinitely. We note their total area :
γ
The two white circles are the biggest circles we can inscribe between the incircle of the big triangle and the largest hexagon. We note the area of the two white circles :
ϕ
The two white squares are inscribed between the incircle of the largest triangle and the largest hexagon. We note the area of the two white squares :
θ
THE QUESTION :
Evaluate :
ϕ
×
γ
α
×
θ
+
β
The answer can be written in its simplest form as
e
×
π
f
a
+
b
c
+
d
π
Calculate the value
k
=
3
a
−
c
b
+
f
×
e
⌊
π
⌋
−
⌈
π
⌉
d
Finally, what is the sum (rounded to the nearest integer) of all the solutions to
2
c
o
s
(
2
x
)
=
−
2
if
5
−
k
π
<
x
<
π
(
3
+
k
)
The answer is 518.
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To find the value of
β
we need to realise that we have a geometric progression. To get the next term of the series we need to multiply the number of circles by 3 and divide their radius by 3 since the angles are 60°. So
β
=
n
=
1
∑
∞
π
⋅
(
6
3
⋅
3
n
1
)
2
⋅
3
n
=
2
4
π
Next let's find
θ
. The diagonale of a white square is equal to the radius of the triangle's incircle minus half the height of the largest hexagon.
The diagonal is
6
3
−
6
3
⋅
sin
(
6
0
°
)
=
1
2
2
3
−
3
Finally the calculate the total area of the two squares :
θ
=
(
1
2
2
3
−
3
)
2
=
4
8
7
−
4
3
Now we deal with
ϕ
, the diameter of the white circles is equal to the white square's diagonal, then
ϕ
=
2
⋅
π
⋅
(
2
4
2
3
−
3
)
2
=
9
6
π
(
7
−
4
3
)
Now let's evaluate
γ
. Using basic trigonometry we find that the first black circle will be inscribed in an isosceles triangle (bright blue) of side length
8
1
,
8
1
and
8
3
. The radius of a triangle's incircle is given by the formula :
r
=
p
e
r
i
m
e
t
e
r
2
×
A
r
e
a
which gives
r
=
4
1
+
8
3
(
8
1
)
2
sin
(
6
0
∘
)
=
1
6
2
3
−
3
. Using the same method to calculate the radius of the next black circle, we realise that to get the next radius you need to multiply the last radius by
4
3
This means we have a geometric progression. The total area of the black circles is given by the formula
γ
=
n
=
0
∑
∞
2
π
⋅
(
1
6
2
3
−
3
⋅
(
4
3
)
n
)
2
=
5
6
π
(
2
1
−
1
2
3
)
Finally let's get the value of
α
. We need to calculate
c
the side of the first black square. For that, we need to remember that the angles of the first isosceles triangle are 30°, 30° and 120° and sides
8
1
,
8
1
and
8
3
; we will consider a right triangle inside the isosceles triangle with legs
c
and
2
8
3
−
c
, knowing that the
c
o
t
(
3
0
°
)
=
3
we can solve the following equation :
3
1
=
2
8
3
−
c
c
<=>
c
=
8
8
6
−
3
We use a similar method to get the side of the next black square and we realise that to get the side of the next black square we need to multiply the last side by
4
3
This means we have a geometric progression. The total area of the black squares is given by the formula
α
=
n
=
0
∑
∞
2
⋅
(
8
8
6
−
3
⋅
(
4
3
)
n
)
2
=
1
6
9
4
3
9
−
1
2
3
This means the interval is − 2 π < x < 1 3 π I give full credit to this solution by brilliant staff
The approximation of 1 6 5 π ≈ 5 1 8