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# Common Mistakes in Geometry Questions - Exercise Question #4

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e-GMAT Representative
Joined: 04 Jan 2015
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Common Mistakes in Geometry Questions - Exercise Question #4 [#permalink]

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22 Nov 2016, 04:33
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Difficulty:

95% (hard)

Question Stats:

39% (01:17) correct 61% (01:35) wrong based on 170 sessions

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Common Mistakes in Geometry Questions - Exercise Question #4

A bigger circle (with center A) and a smaller circle (with center B) are touching each other externally. PT and PS are the tangents drawn to these circles from an external point (as shown in the figure). What is the length of ST?

(1) The radii of the bigger and the smaller circles are 9 cm and 4 cm respectively
(2) PB = 52/5 cm

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.

B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.

C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.

D. EACH statement ALONE is sufficient.

E. Statements (1) and (2) TOGETHER are NOT sufficient.

Common Mistakes in Geometry are explained in detail in the following post:

Common Mistakes in Geometry Questions

Detailed solution will be posted soon.

[Reveal] Spoiler: OA

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Last edited by EgmatQuantExpert on 02 Dec 2016, 08:17, edited 2 times in total.

Kudos [?]: 2322 [0], given: 126

e-GMAT Representative
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Kudos [?]: 2322 [3], given: 126

Re: Common Mistakes in Geometry Questions - Exercise Question #4 [#permalink]

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22 Nov 2016, 10:11
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Note: This questions is related to the article on Common Errors in Geometry

Kindly go through the article once, before solving the question or going through the solution.

Official Solution

Steps 1 & 2: Understand Question and Draw Inferences

Given that PT is a tangent to the small circle and PS is a tangent to the big circle.
ΔPTB and ΔPSA are right angled at T and S respectively.
We need to find the length of ST.

Step 3: Analyze Statement 1

(1) The radii of the bigger and the smaller circles are 9 cm and 4 cm respectively.
If we drop a perpendicular BD on side AS, we get:

BDST is a rectangle (since all angles of this quadrilateral are right angles).
Therefore, since opposite sides of a rectangle are equal, SD = BT = 4 cm
This means, AD = 9 – 4 = 5cm
In right triangle ADB, by applying Pythagoras Theorem, we get:
B$$D^2$$ + A$$D^2$$ = A$$B^2$$

That is, B$$D^2$$ = (9+4$$)^2$$ – (5$$)^2$$

B$$D^2$$ = (13+5)(13-5)

Therefore, BD = 12

Since opposite sides of a rectangle are equal, ST = BD = 12 cm
Since we have been able to determine a unique length of ST, Statement (1) is sufficient.

Step 4: Analyze Statement 2

(2) PB = 52/5 cm

In right triangle BTP, we know the length of only one side: BP.
In order to find the lengths of the other sides of this triangle, we need one more piece of information – either one of the two unknown angles, or one of the two unknown sides.
Since we don’t have this information, we will not be able to find the lengths of the unknown sides of triangle BTP.
Due to a similar reasoning, we will not be able to find the length of the sides of triangle ASP.
So, we will not have enough information to find the length of ST.

Therefore statement 2 is not sufficient to arrive at a unique answer.

Since we got the answer from the first statement, Correct Answer: A

Thanks,
Saquib
Quant Expert
e-GMAT

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Last edited by EgmatQuantExpert on 01 Dec 2016, 00:58, edited 1 time in total.

Kudos [?]: 2322 [3], given: 126

e-GMAT Representative
Joined: 04 Jan 2015
Posts: 786

Kudos [?]: 2322 [0], given: 126

Re: Common Mistakes in Geometry Questions - Exercise Question #4 [#permalink]

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27 Nov 2016, 09:43
Hey Everyone,

The official solution has been posted. Kindly go through it and if you have any doubts feel free to post your query.

Thanks,
Saquib
Quant Expert
e-GMAT
_________________

| '4 out of Top 5' Instructors on gmatclub | 70 point improvement guarantee | www.e-gmat.com

Kudos [?]: 2322 [0], given: 126

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Re: Common Mistakes in Geometry Questions - Exercise Question #4 [#permalink]

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28 Dec 2017, 07:46
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Re: Common Mistakes in Geometry Questions - Exercise Question #4   [#permalink] 28 Dec 2017, 07:46
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