In square A B C D , four congruent equilateral triangles are wedged in to form the inner red square.
Let O be the center of the red square and point Q a vertex of the red square and fold O Q at a right angle, as shown above, to form the height of the right square pyramid.
Let V p be the volume of the right square pyramid and V C be the volume of the cube formed using square A B C D .
If V C V P = λ γ ( α − β λ ) γ λ , where α , β . λ and γ are coprime positive integers, find α + β + λ + γ .
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A B C D be a unit square. Then, the volume of the cube is V C = 1 .
For simplicity let
Now, with the labelling of the figure where
a
is the side length of the equilateral triangles and
s
is the side length of the inner red square, we focus on
△
F
E
B
:
∠
F
E
B
=
1
2
0
∘
and
∠
F
B
E
=
3
0
∘
, hence
∠
E
F
B
=
3
0
∘
, i..e.
△
F
E
B
is isosceles, with
E
B
=
E
F
=
1
−
a
.
By sine rule, sin E F B = sin B F E ⇒ sin 1 2 0 ∘ a = sin 3 0 ∘ 1 − a ⇒ 2 1 a = 2 3 ( 1 − a ) ⇒ a = 2 3 − 3 Furthermore, s = G F = G E − F E = a − ( 1 − a ) = 2 a − 1 = 2 2 3 − 3 − 1 ⇒ s = 2 − 3 For the height h of the pyramid, h = O Q = 2 G Q = 2 s 2 = 2 s Thus, the volume of the pyramid is V P = 3 1 s 2 h = 3 1 ⋅ 2 s 3 = 3 2 ( 2 − 3 ) 3 = 3 2 [ ( 2 − 3 ) 2 ] 2 3 = 3 2 ( 7 − 4 3 ) 2 3 ⇒ V C V P = 1 V P = 3 2 ( 7 − 4 3 ) 2 3 Hence, α = 7 , β = 4 , λ = 3 , γ = 2 and α + β + λ + γ = 7 + 4 + 3 + 2 = 1 6 .
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Let m be a side of square A B C D .
Let A e be the area of equilateral △ D F E ⟹ 4 A e = 4 ( 2 1 ) ( a ) ( 2 3 a ) = 3 a 2
Let A I be the area of △ G F D
The height h △ G F D = ( m − a ) sin ( 6 0 ∘ ) = 2 3 ( m − a ) ⟹
4 A I = 4 ( 2 1 ) ( m − a ) 2 ( 2 3 ) = 3 ( m − a ) 2
Using the law of cosines on △ G F D with included ∠ F G D ⟹
a 2 = 2 ( m − a ) 2 ( 2 3 ) = 3 ( m − a ) 2 = 3 a 2 − 6 m a + 3 m 2 ⟹ 2 a 2 − 6 m a + 3 m 2 = 0
⟹ a = 2 3 ± 3 m
a = 2 3 + 3 m ⟹ m − a = − ( 1 + 3 ) m < 0
a = 2 3 − 3 m ⟹ m − a = 2 3 − 1 m
∴ we drop a = 2 3 + 3 m and choose a = 2 3 − 3 m
⟹ 4 A e = 4 3 ( 3 − 3 ) 2 m 2 = 2 3 3 ( 2 − 3 ) m 2
and
4 A I = 3 ( m − a ) 2 = 4 3 ( 3 − 1 ) 2 m 2 = 2 3 ( 2 − 3 ) m 2
Let A s be the area of the red square
⟹ A s = m 2 − ( 4 A e + 4 A I ) = ( 1 − ( 2 3 ( 2 − 3 ) ) ) m 2 = ( 1 − ( 4 3 − 6 ) ) m 2
= ( 7 − 4 3 ) m 2
Let y be a side of the red square ⟹ y = 7 − 4 3 m ⟹ O Q = 2 2 7 − 4 3 m = 2 7 − 4 3 m ⟹
V p = 3 1 A s O Q = 3 2 ( 7 − 4 3 ) 2 3 m 3 ⟹
V C V P = 3 2 ( 7 − 4 3 ) 2 3 = λ γ ( α − β λ ) γ λ ⟹
α + β + λ + γ = 1 6 .