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Re: M18-15 [#permalink]
I think this is a high-quality question and I agree with explanation.
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Re: M18-15 [#permalink]
Bunuel wrote:
Official Solution:

In rectangle \(ABCD\), \(E\) is the point of intersection of diagonals. If angle \(ABD\) is twice angle \(EAD\), what is the value of angle \(CED\)?

A. 30 degrees
B. 45 degrees
C. 60 degrees
D. 90 degrees
E. 120 degrees


Look at the diagram below:



Notice that since \(\angle{BAD}=90\) degrees then \(\angle{EAB}=90-x\) degrees.

Next, since diagonals of a rectangle are equal and bisect each other then \(AE=BE\). So, \(\angle{EAB}=\angle{EBA}\): \(90-x=2x\), which gives \(x=30\). Thus, \(\angle{EAB}=\angle{EBA}=60\) and \(\angle {AEB}=60\) degrees.

Finally as \(\angle{AEB}=\angle{CED}\) then \(\angle{CED}=60\) degrees.


Answer: C



Hi,
Aren't the diagonals of a rectangle an angular bisector ? AS, WE KNOW THAT DE = BE. So, angle a should have each angle = 45 degree ? angle DAE = angle BAE = 45 ?
thanks
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Re: M18-15 [#permalink]
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19941010 wrote:
Bunuel wrote:
Official Solution:

In rectangle \(ABCD\), \(E\) is the point of intersection of diagonals. If angle \(ABD\) is twice angle \(EAD\), what is the value of angle \(CED\)?

A. 30 degrees
B. 45 degrees
C. 60 degrees
D. 90 degrees
E. 120 degrees


Look at the diagram below:



Notice that since \(\angle{BAD}=90\) degrees then \(\angle{EAB}=90-x\) degrees.

Next, since diagonals of a rectangle are equal and bisect each other then \(AE=BE\). So, \(\angle{EAB}=\angle{EBA}\): \(90-x=2x\), which gives \(x=30\). Thus, \(\angle{EAB}=\angle{EBA}=60\) and \(\angle {AEB}=60\) degrees.

Finally as \(\angle{AEB}=\angle{CED}\) then \(\angle{CED}=60\) degrees.


Answer: C



Hi,
Aren't the diagonals of a rectangle an angular bisector ? AS, WE KNOW THAT DE = BE. So, angle a should have each angle = 45 degree ? angle DAE = angle BAE = 45 ?
thanks


The diagonal of a rectangle is a bisector only if the rectangle is a square.
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Re: M18-15 [#permalink]
Aren't the diagonals of rectangle perpendicular bisector of each other?
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Re: M18-15 [#permalink]
Expert Reply
La1yaMalhotra wrote:
Aren't the diagonals of rectangle perpendicular bisector of each other?


The diagonals of a rectangle always bisect each other but they are perpendicular to each other only if a rectangle is a square.
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Re: M18-15 [#permalink]
The answer is option C. Well explained above.
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Re: M18-15 [#permalink]
I think this is a high-quality question and I agree with explanation.
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Re: M18-15 [#permalink]
Can you please explain the angle measure 2x for EBA
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Re: M18-15 [#permalink]
I think this is a high-quality question and I agree with explanation. Good & helpful.
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Re: M18-15 [#permalink]
Good question.

I think what this brings to light are the rules of diagonals in a rectangle and the difference between the bisector of a rectangle and square.

"Each diagonal of a square is the perpendicular bisector of the other. That is, each cuts the other into two equal parts, and they cross and right angles (90°)"

Whereas, each diagonal of a rectangle bisects each other and divides the rectangle up into two equal parts, but the point at which the diagonals bisect does not create 4 equal angles.

Then, finding out what angle corresponds to what side can allow us to solve 90-x = 2x and so forth.

Thanks Bunuel
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Re: M18-15 [#permalink]
I think this is a high-quality question and I agree with explanation.
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Re: M18-15 [#permalink]
Great points in this question:

1. The diagonal of a rectangle is a bisector only if the rectangle is a square.
2. The diagonals of a rectangle always bisect each other but they are perpendicular to each other only if a rectangle is a square.

I made the mistake of assuming the diagonal as a bisector and selected the D option.

Great explanation with great information.
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Re: M18-15 [#permalink]
Expert Reply
I have edited the question and the solution by adding more details to enhance its clarity. I hope it is now easier to understand.
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Re: M18-15 [#permalink]
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