is a rectangle with center . is a point on such that is a rectangle with center such that lies on . is a rectangle with center such that lies on . What is the measure (in degrees) of ?
Details and assumptions
The center of a rectangle is the intersection of the 2 diagonals.
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1.Observation: It is easy to check that A E B H and A E C F are cyclic. J , K , L are the mid-points of the diagonals ( A C , B D ) , A F , D H of A B C D , A E F D and D E H I . ∴ J K ∣ ∣ C F and J L ∣ ∣ B H .
2.Angle Chasing: (Draw a clear picture to follow the chasing) ∠ D J L = ∠ D B H = ∠ D B A + ∠ A B H = ∠ D B A + ∠ A E H = ∠ D B A + ( 9 0 ∘ − ∠ H A E ) = ∠ D B A + ( 9 0 ∘ − ∠ A E D ) [ ∵ I H ∣ ∣ D E ] = ∠ D B A + ( 9 0 ∘ − 2 5 ∘ ) = ∠ D B A + 6 5 ∘
Similarly, ∠ A J K = ∠ A C D + 6 5 ∘
Now, ∠ A J K + ∠ D J L = ( ∠ A J D + ∠ D J K ) + ( ∠ D J A + ∠ A J L ) = ( ∠ A J D + ∠ D J K + ∠ A J L ) + ∠ D J A = ∠ K J L + ∠ D J A .
∴ ∠ K J L = ( ∠ A J K + ∠ D J L ) − ∠ D J A = ( 1 3 0 ∘ + ∠ D B A + ∠ A C D ) − ∠ D J A = 1 3 0 ∘ + ( 2 ∠ D B A − ∠ D J A ) = 1 3 0 ∘ .