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MathRevolution
[GMAT math practice question]

A \(30^o-60^o-90^o\) triangle is drawn on the exterior of equilateral triangle ABC as shown in the figure below so that the hypotenuse of the right triangle forms one side of the equilateral triangle. If the length of CD is \(2\), what is the length of AD?

Attachment:
6.10.png

A. 3
B. 4
C. 5
D. √7
E. 2√7

One way to solve it.

Extend a line parallel to \(BD\) from the point \(A\) and mark the other end as \(F\). \(AF\) will be parallel to \(BD\) with length as \(2 \sqrt{3}\).

Now it becomes a \(RECTANGLE\) with \(EQUILATERAL TRIANGLE\) in it with the base as length

AB will be \(4\), which will the length of the rectangle and width as \(2 \sqrt{3}\)

Diagonal of the rectangle will be \(2 \sqrt{7}\)
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MathRevolution
[GMAT math practice question]

A \(30^o-60^o-90^o\) triangle is drawn on the exterior of equilateral triangle ABC as shown in the figure below so that the hypotenuse of the right triangle forms one side of the equilateral triangle. If the length of CD is \(2\), what is the length of AD?

Attachment:
6.10.png

A. 3
B. 4
C. 5
D. √7
E. 2√7

using 30;60;90
we get side to 90 ; 4
so
height of ∆ ; 2√3
∆ADB ; AD = (2√3)^2+(4^2 = √28
=2√7
IMO E
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=>

Since the triangle \(BCD\) is a \(30-60-90\) triangle, \(CD:BC:BD = 1:2: √3.\)

So, \(BC = 4, AB = 4,\) and \(BD = 2 √3.\)

Angle \(ABD\) is a right angle since angle \(ABC\) is \(60\) degrees, and angle \(CBD\) is \(30\) degrees.

By Pythagoras’ theorem, \(AD2 = AB2 + BD2 = 16 + 12 = 28\). Therefore, \(AD = 2√7.\)

Therefore, E is the answer.
Answer: E
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