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# In the figure above, what is the length of AB?

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In the figure above, what is the length of AB?  [#permalink]

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23 Oct 2019, 22:15
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25% (medium)

Question Stats:

67% (00:48) correct 33% (00:47) wrong based on 60 sessions

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In the figure above, what is the length of AB?

(A) 5
(B) 7
(C) 2√7
(D) 4√2
(E) 10

Attachment:

2017-08-08_2111_001.png [ 3.7 KiB | Viewed 633 times ]

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Re: In the figure above, what is the length of AB?  [#permalink]

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23 Oct 2019, 22:53
Bunuel wrote:

In the figure above, what is the length of AB?

(A) 5
(B) 7
(C) 2√7
(D) 4√2
(E) 10

Attachment:
2017-08-08_2111_001.png

$$AB = 2\sqrt{4^2 - 3^2} = 2\sqrt{7}$$

IMO C
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Re: In the figure above, what is the length of AB?  [#permalink]

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31 Oct 2019, 04:55

Solution

Given:
A triangle ABC,
AC = BC = 4 units
And the length of perpendicular dropped from C on AB = 3 units

To find:
The length of AB

Approach and Working Out:
Let us assume that that the foot of the perpendicular as D

In triangle ACD,
$$AC^2 = AD^2 + CD^2$$
$$4^2 = AD^2 + 3^2$$
Implies, $$AD^2 = 16 – 9 = 7$$

Similarly, we can find DB = √7

Therefore, AB = AD + DB = 2√7

Hence, the correct answer is Option C.

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Re: In the figure above, what is the length of AB?  [#permalink]

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13 Nov 2019, 05:40
Why are those not two 3-4-5 triangles?
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Re: In the figure above, what is the length of AB?  [#permalink]

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13 Nov 2019, 05:47
1
Moritz279 wrote:
Why are those not two 3-4-5 triangles?

In a right triangle with side lengths of 3, 4, and 5, 5 is the length of the longest side, so the length of the hypotenuse. So, let me ask you, is that so in the given figure?
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Re: In the figure above, what is the length of AB?  [#permalink]

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18 Nov 2019, 19:35
Hello Bunuel,

Why can't the answer be 4 sqrt(2) here? The way i reached the solution was consider the entire triangle as an isosceles triangle and AB as the hypotenuse and angle ACB = 90 deg.

Thank you
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Re: In the figure above, what is the length of AB?  [#permalink]

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18 Nov 2019, 19:51
SiddharthR wrote:
Hello Bunuel,

Why can't the answer be 4 sqrt(2) here? The way i reached the solution was consider the entire triangle as an isosceles triangle and AB as the hypotenuse and angle ACB = 90 deg.

Thank you

Sorry my bad. Just realized that just because the sides are equal it doesn't mean the angles would be 45 - 45. Pretty big blunder on my part.
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Re: In the figure above, what is the length of AB?  [#permalink]

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18 Nov 2019, 22:47
This is a fairly easy question on the properties of isosceles and right angled triangles.

Triangle ABC is an isosceles triangle with AC = CB. Therefore, AB is the unequal side and angle ACB is the unequal angle. Let D be the point where the perpendicular from C meets AB. Then, triangles ACD and triangles BCD are right angled triangles with the right angle at D.

In an isosceles triangle, the perpendicular drawn from the vertex containing the unequal angle bisects the unequal side. Additionally, it also bisects the unequal angle. In short, this line acts as the perpendicular bisector of the unequal side and as the angle bisector of the unequal angle.

Therefore, in the isosceles triangle ABC, AD = DB. In right angled triangle ACD, the hypotenuse AC = 4 and the perpendicular CD = 3. Using Pythagoras theorem, $${AC}^2 = {AD}^2 + {CD}^2$$. Substituting the values of AC and CD in the equation above, we can calculate AD to be √7.

Since triangle BCD is exactly the same as triangle ACD (which is to say that triangle ACD is congruent with congruent BCD), it’s not hard to figure out that DB will also be √7.
AB = AD + DB. Therefore, AD = √7+ √7 = 2√7.
The correct answer option is C.

The fact that AD and DB are equal also ties in with angle bisector theorem. The angle bisector theorem states “The angle bisector of an interior angle of a triangle divides the opposite side in the ratio of the arms of the angle bisected by it”.

In this question, CD is the angle bisector of inteior angle ACB (as per the property of an isosceles triangle). The arms of this angle ACB are AC and CB which are equal and hence in the ratio of 1:1. You can now observe that the angle bisector is bisecting the opposite side AB in the ratio of 1:1 i.e. AD = DB, which is what we proved.

Hope that helps!
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Re: In the figure above, what is the length of AB?  [#permalink]

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20 Nov 2019, 19:22
Bunuel wrote:

In the figure above, what is the length of AB?

(A) 5
(B) 7
(C) 2√7
(D) 4√2
(E) 10

Attachment:
2017-08-08_2111_001.png

Both triangles have the same angles, height, and hypotenuse, so they must also have the same base. We can solve for one base and multiply it by 2. So, we have:

3^2 + b^2 = 4^2

b^2 = 7

b = √7

Thus, AB = 2√7.

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Re: In the figure above, what is the length of AB?   [#permalink] 20 Nov 2019, 19:22
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