In square A B C D with side length 1 and diagonal A C and E C , the two red circles are congruent with radius r and tangent to the sides of △ A C E and △ E C D as shown above.
If the radius r can be expressed as r = ω λ − ω − λ , where λ and ω are coprime positive integers, find λ + ω .
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A △ E C D = 2 1 y = 2 1 r ( a + y + 1 ) ⟹ r = a + y + 1 y
and
A △ A C E = 2 1 r ( a + 2 + 1 − y ) = 2 1 ( 1 − y ) ⟹
r = a + 2 + 1 − y 1 − y = a + y + 1 y ⟹ ( 2 a + 2 + 1 ) y = a + 1 ⟹
y = 2 a + 2 + 1 a + 1
In △ E C D ⟹ y 2 + 1 = a 2 ⟹ ( a + 1 ) 2 + ( 2 a + 2 + 1 ) 2 = a 2 ( 2 a + 2 + 1 ) 2
⟹ ( a + 1 ) ( a + 1 − ( a − 1 ) ( 2 a + 2 + 1 ) 2 ) = 0 and a = 1 ⟹
2 a 3 + 2 2 a 2 − ( 2 + 1 ) a − 2 ( 2 + 1 ) = 0 ⟹
2 a 2 ( a + 2 ) − ( 2 + 1 ) ( a + 2 ) = 0 ⟹ ( 2 a 2 − ( 2 + 1 ) ) ( a + 2 ) = 0
and a = − 2 ⟹ a 2 = 2 2 + 1 ⟹ a = 2 2 + 1
⟹ y 2 + 1 = 2 2 + 1 ⟹ y = 2 2 − 1
Let α = 2 + 1 and β = 2 − 1 ⟹ a = 2 α and y = 2 β ⟹
r = a + y + 1 y = α + β + 2 β = 4 2 β ( α + β − 2 )
= 4 2 ( 1 + 2 − 1 − 2 β ) = 4 2 ( 1 − β ) =
2 1 − 2 − 1 = ω λ − ω − λ ⟹ λ + ω = 3 .
By the properties of a unit square, A D = C D = 1 and A C = 2 . Let E D = x , so that A E = A D − E D = 1 − x , and E C = E D 2 + C D 2 = x 2 + 1 .
Then the areas of △ A E C and △ E D C are A △ A E C = 2 1 ⋅ A E ⋅ C D = 2 1 ⋅ ( 1 − x ) ⋅ 1 = 2 1 ( 1 − x ) and A △ E D C = 2 1 ⋅ E D ⋅ C D = 2 1 ⋅ x ⋅ 1 = 2 1 x .
And the perimeters are P △ A E C = A E + A C + E C = 1 − x + 2 + x 2 + 1 and P △ E D C = E D + C D + E C = x + 1 + x 2 + 1 .
Since the inradius is r = P 2 A , then r = P △ A E C 2 A △ A E C = P △ E D C 2 A △ E D C , or r = 1 − x + 2 + x 2 + 1 1 − x = x + 1 + x 2 + 1 x , which solves to x = 2 1 ( 2 − 1 ) .
Substituting x = 2 1 ( 2 − 1 ) into r = x + 1 + x 2 + 1 x and solving gives r = 2 1 − 2 − 1 , so λ = 1 , ω = 2 , and λ + ω = 3 .
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Let the centers of the left and right circles be O and P respectively, O M and P N be perpendicular to A D , P L is perpendicular to C D , and ∠ E C D = θ . Then ∠ C E D = 9 0 ∘ − θ and ∠ C E A = 9 0 ∘ + θ . Note that ∠ C A D = 4 5 ∘ , and C P , E P , O E , and A O bisect ∠ E C D , ∠ C E D , ∠ C E A , and ∠ C A D respectively.
We can apply half-angle tangent substitution to solve triangle incircle problems reducing them to solving polynomial equations. From
C L + L D r cot 2 θ + r t r + r ⟹ t = C D = 1 = 1 = 1 − r r Let t = tan 2 θ
From:
A M + M E + E N + N D r cot 2 2 . 5 ∘ + r cot ( 4 5 ∘ + 2 θ ) + r cot ( 4 5 ∘ − 2 θ ) + r ( 2 + 1 ) r + 1 + t 1 − t ⋅ r + 1 − t 1 + t ⋅ r + r ( 2 + 2 ) r + ( 1 − 2 r ) r + 1 − 2 r r 4 r 3 − ( 8 + 2 2 ) r 2 + ( 6 + 2 ) r − 1 ⟹ r = A D = 1 = 1 = 1 = 0 = ⎩ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎧ 1 + 2 1 2 1 − 2 − 1 2 1 + 2 − 1 Note that t = 1 − r r for r = 2 1
The acceptable value is r = 2 1 − 2 − 1 . Therefore λ + ω = 1 + 2 = 3 .