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By angle chasing, we find that ∠ B A E = 2 5 ∘ and ∠ A E B = 6 5 ∘ . Then ∠ F E C = 1 8 0 ∘ − 6 5 ∘ − x ∘ = 1 1 5 ∘ − x ∘ and
tan ( 1 1 5 ∘ − x ∘ ) 1 1 5 ∘ − x ∘ ⟹ x = C E F C = 1 − tan 2 5 ∘ 1 − tan 2 0 ∘ = 1 − tan 2 5 ∘ 1 − tan ( 4 5 ∘ − 2 0 ∘ ) = 1 − tan 2 5 ∘ 1 − 1 + tan 2 5 ∘ 1 − tan 2 5 ∘ = 1 − tan 2 2 5 ∘ 2 tan 2 5 ∘ = tan 5 0 ∘ = 5 0 ∘ = 6 5 Let A B = B C = C D = D A = 1
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consider triangle ADF and ABE (sorry to say i still don't know how to answer which look beautiful enough in the way of math) AD=BD cos(20) AF = cos(25) AE
A E A F = c o s ( 2 0 ) c o s ( 2 5 ) for triangle AEF take a perpendicular to EF which go though A. now we have two triangles AOE and AOF
so AO = sin(x) AE and AO = sin(y) AF
A E A F = s i n ( y ) s i n ( x ) = c o s ( 2 0 ) c o s ( 2 5 ) = s i n ( 7 0 ) s i n ( 6 5 )
y x = 7 0 6 5
x= 65k ( k is a constant) for triangle k=1
so x = 65