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• ### $450 Tuition Credit & Official CAT Packs FREE December 15, 2018 December 15, 2018 10:00 PM PST 11:00 PM PST Get the complete Official GMAT Exam Pack collection worth$100 with the 3 Month Pack ($299) # In the xy-plane, is the triangle that connects the 3 different points  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics Author Message TAGS: ### Hide Tags Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 6639 GMAT 1: 760 Q51 V42 GPA: 3.82 In the xy-plane, is the triangle that connects the 3 different points [#permalink] ### Show Tags 15 Jul 2018, 05:58 1 3 00:00 Difficulty: 85% (hard) Question Stats: 31% (02:11) correct 69% (01:46) wrong based on 26 sessions ### HideShow timer Statistics [GMAT math practice question] In the xy-plane, is the triangle that connects the 3 different points $$A(3,4), B(p,q)$$, and $$C(r,s)$$ a right triangle? 1) $$(p-3)(r-3)+(q-4)(s-4)=0$$ 2) $$p=3$$ and $$s=4$$ _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$99 for 3 month Online Course"
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Re: In the xy-plane, is the triangle that connects the 3 different points  [#permalink]

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15 Jul 2018, 06:34
1
MathRevolution wrote:
[GMAT math practice question]

In the xy-plane, is the triangle that connects the 3 different points $$A(3,4), B(p,q)$$, and $$C(r,s)$$ a right triangle?

1) $$(p-3)(r-3)+(q-4)(s-4)=0$$
2) $$p=3$$ and $$s=4$$

Nice question!!

Here is my approach for this problem.

Stat 1: Here lets try to reduce the the equation in slope form(We know for perpendicular lines product of slope is -1)
So (p-3)(r-3)+(q-4)(s-4)=0
(p-3)(r-3)=-(q-4)(s-4)=0
(p-3)/(q-4)=-(r-3)/(s-4)

Now vertex of triangle is given as A(3,4),B(p,q) and C(r,s)
So clearly from above equation we see lines AB and AC are perpendicular.

Stat2:

P=3 implies point B lies in the horizaontal line that passes through point A for which also X coordinate is 3.
s=4 implies point C lies in the vertical line that passes through point A for which also Y coordinate is 4.
So both lines will be perpindicular at A and hence a right angle triangle.

Option D is correct choice.

Press Kudos if it helps!!
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GMAT 1: 760 Q51 V42
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Re: In the xy-plane, is the triangle that connects the 3 different points  [#permalink]

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15 Jul 2018, 17:16
=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.

Condition 2):
If $$p = 3$$, then $$q ≠ 4$$ since $$(p,q)$$ is different from $$(3,4).$$
If $$s = 4$$, then $$r ≠ 3$$ since $$(r,s)$$ is different from $$(3,4)$$.
So, $$(p,q)$$ is on the line $$x = 3$$ and $$(r,s)$$ is on the line $$y = 4.$$
As these lines are perpendicular AB and AC are perpendicular, and triangle ABC is a right triangle.
Thus, condition 2) is sufficient.

Condition 1) is complicated. If you can’t figure out how to apply condition 1), CMT4(B) tells you to choose D as the answer.

Condition 1)
$$(p-3)(r-3)+(q-4)(s-4)=0$$
$$=> (p-3)(r-3) = -(q-4)(s-4)$$
$$=> \frac{(p-3)(r-3)}{(q-4)(s-4)} = -1$$ or $$(q-4)(s-4) = 0$$
$$=> {\frac{(p-3)}{(q-4)}} {\frac{(r-3)}{(s-4)}} = -1$$ or $$q = 4$$ or $$s = 4$$

Case 1: $$\frac{(p-3)}{(q-4)} * {\frac{(r-3)}{(s-4})} = -1$$
$$\frac{(p-3)}{(q-4)}$$ and $$\frac{(r-3)}{(s-4)}$$ are the slopes of two sides of the triangle.
Since the product of these slopes is $$-1$$, the two sides are perpendicular and the triangle is a right triangle.

Case 2: $$q = 4$$
If $$q = 4$$, then $$p$$ is not $$3$$. Also, we must have $$(p-3)(r- 3) = 0$$. So, $$r = 3$$ since $$(p,q)$$ is different from $$(3,4)$$.
Thus, $$(p,q)$$ lies on the line $$y = 4$$ and $$(r,s)$$ lies on the line $$x = 3$$.
As these lines are perpendicular, AB and AC are perpendicular, and triangle ABC is a right triangle.

Case 3: $$s = 4.$$
A similar argument to the one used for case 2 shows that triangle ABC is a right triangle.

Thus, condition 1) is also sufficient.

In cases where 3 or more additional equations are required, such as for original conditions with “3 variables”, or “4 variables and 1 equation”, or “5 variables and 2 equations”, conditions 1) and 2) usually supply only one additional equation. Therefore, there is an 80% chance that E is the answer, a 15% chance that C is the answer, and a 5% chance that the answer is A, B or D. Since E (i.e. conditions 1) & 2) are NOT sufficient, when taken together) is most likely to be the answer, it is generally most efficient to begin by checking the sufficiency of conditions 1) and 2), when taken together. Obviously, there may be occasions on which the answer is A, B, C or D.
_________________

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Re: In the xy-plane, is the triangle that connects the 3 different points &nbs [#permalink] 15 Jul 2018, 17:16
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