A Few Rectangles

Geometry Level 4

A B C D ABCD is a rectangle with center J J . E E is a point on B C BC such that A E D = 2 5 . \angle AED = 25 ^\circ . A E F G AEFG is a rectangle with center K K such that D D lies on F G FG . D E H I DEHI is a rectangle with center L L such that A A lies on H I HI . What is the measure (in degrees) of K J L \angle KJL ?

Details and assumptions

The center of a rectangle is the intersection of the 2 diagonals.


The answer is 130.

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1 solution

Sowmitra Das
Dec 19, 2013

1.Observation: It is easy to check that A E B H AEBH and A E C F AECF are cyclic. J , K , L J, K, L are the mid-points of the diagonals ( A C , B D ) (AC, BD) , A F , D H AF, DH of A B C D , A E F D ABCD, AEFD and D E H I DEHI . J K C F \therefore JK||CF and J L B H JL||BH .

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 \angle DJL=\angle DBH=\angle DBA+\angle ABH=\angle DBA+\angle AEH = D B A + ( 9 0 H A E ) = D B A + ( 9 0 A E D ) =\angle DBA+(90^\circ-\angle HAE)=\angle DBA+(90^\circ-\angle AED) [ I H D E ] [\because IH||DE] = D B A + ( 9 0 2 5 ) = D B A + 6 5 =\angle DBA+(90^\circ-25^\circ)=\angle DBA+65^\circ

Similarly, A J K = A C D + 6 5 \angle AJK=\angle ACD+65^\circ

Now, A J K + D J L = \angle AJK+\angle DJL= ( A J D + D J K ) + ( D J A + A J L ) (\angle AJD+\angle DJK)+(\angle DJA+\angle AJL) = ( A J D + D J K + A J L ) + D J A = =(\angle AJD+\angle DJK+\angle AJL)+\angle DJA= K J L + D J A . \angle KJL+\angle DJA.

K J L = ( A J K + D J L ) D J A \therefore \angle KJL=(\angle AJK+\angle DJL)-\angle DJA = ( 13 0 + D B A + A C D ) D J A = =(130^\circ+\angle DBA+\angle ACD)-\angle DJA= 13 0 + ( 2 D B A D J A ) = 13 0 130^\circ+(2\angle DBA-\angle DJA)=\boxed{130^\circ} .

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