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Set S contains nine distinct points in the coordinate plane. If exactl

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Set S contains nine distinct points in the coordinate plane. If exactl  [#permalink]

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New post 28 Apr 2017, 02:02
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Set S contains nine distinct points in the coordinate plane. If exactly five of the points lie on the x axis, and if no other set of three points in S is collinear, how many triangles can be formed by taking points in S as vertices?

A. 56
B. 70
C. 74
D. 79
D. 84

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Re: Set S contains nine distinct points in the coordinate plane. If exactl  [#permalink]

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New post 28 Apr 2017, 02:31
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Given information : Out of 9 points, 5 points lie on the same line.
We can form a triangle using 3 points, so the total number of triangles possible are 9c3 = 84
But no triangles can be formed among the 5 points(5c3 = 10)
Hence total triangles possible is 74(84 - 10) (Option C)
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Re: Set S contains nine distinct points in the coordinate plane. If exactl  [#permalink]

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New post 02 May 2017, 03:50
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first consider the scenario with no restrictions..
total points = 9
triangles possible = 9c3 = 84
since five of them lie in x axis,,
the possible combination of 5 points with 3 at a time is 5c3 = 10 (triangles not possible)

total possible traingles = 84 - 10 = 74
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Set S contains nine distinct points in the coordinate plane. If exactl  [#permalink]

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New post 16 Jun 2017, 15:33
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Bunuel wrote:
Set S contains nine distinct points in the coordinate plane. If exactly five of the points lie on the x axis, and if no other set of three points in S is collinear, how many triangles can be formed by taking points in S as vertices?

A. 56
B. 70
C. 74
D. 79
D. 84


Real simply- the wording in this question in saying that 5 points have an x value of zero and therefore line on the x-axis therefore these five points are collinear and cannot form a triangle. Secondly, when this question states no other set of three points in S is collinear- all that really means is that of the 4 points left these four points cannot form a straight line.

Number of Triangles Possible without restriction- also in order to understand the subtraction of the restriction consider this example. Freddie, Shaggy, Velma, Daphne, and Scooby are about to hunt down a ghost but only two members of can be chosen for the hunt. But Freddie and Daphne just got into an argument so they refuse to go together. So the total number of combinations that could be made, and remember order doesn't matter, would be 5c2-2c2=9. When we say order doesn't matter well what that means is Freddie and Scooby in a group represent one possibility- it doesn't matter what position they are in FS or SF because that constitutes the same combination. If we just did 9c3 we would count the total number of possible triangle that include triangles made from the five points.

9c3 - 5c3 = 74

Thus
"C"

*Notice- on the diagram we cannot form a straight line with 3 points that include any of the points (I, II, III, IV)
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Set S contains nine distinct points in the coordinate plane. If exactl  [#permalink]

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New post 16 Jun 2017, 17:27
Answer C

1) There are 4!/(3!1)=4 ways of creating triangle out of the 4 points not on the X axis
2) We know take 2 points not on X axis and 1 on X. 4!/(2!2!)= 6 way of picking 2 points. 5!(1!4!)= 5 ways of picking 1 point from X axis. 6*5=30 ways of picking 1 point from X axis, 2 not from there
3) Now lets count number of ways taking 2 points from X axis and 1 not from there. 5!/(3!2!)= 10 ways to pick 2 points. 4!/(1!3!)= 4 wats to pick 1 point. 4*10=40 triangles

Total number of ways= Sum of all possible triangles =40+30+4=74
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Re: Set S contains nine distinct points in the coordinate plane. If exactl  [#permalink]

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New post 16 Jun 2017, 20:57
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In such kind of questions: if there are 'n' total points, out of which 'a' are collinear, the number of triangles formed is always:

nC3 - aC3 , i.e., if all those were collinear then the triangles formed would be nC3, but we have to subtract aC3 to account for those set of points which are not collinear.

So here our answer would be = 9C3 - 5C3 = 84 - 10 = 74.

Thus C is our answer
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Re: Set S contains nine distinct points in the coordinate plane. If exactl  [#permalink]

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New post 30 Oct 2018, 18:13
Bunuel wrote:
Set S contains nine distinct points in the coordinate plane. If exactly five of the points lie on the x axis, and if no other set of three points in S is collinear, how many triangles can be formed by taking points in S as vertices?

A. 56
B. 70
C. 74
D. 79
D. 84


We have 5 (collinear) points that are on the x-axis and 4 points that are not on the x-axis. Since no set of three points in S is collinear except those on the x-axis, we can have the following 3 cases forming a triangle: 1) two points on the x-axis and one point not on the x-axis, 2) one point on the x-axis and two points not on the x-axis, and 3) three points not on the x-axis.

Case 1: Two points on the x-axis and one point not on the x-axis

There are 5C2 x 4C1 = 10 x 4 = 40 such triangles in this case.

Case 2: one point on the x-axis and two points not on the x-axis

There are 5C1 x 4C2 = 5 x 6 = 30 such triangles in this case.

Case 3: Three points not on the x-axis

There are 4C3 = 4 such triangles in this case.

Therefore, there are 40 + 30 + 4 = 74 triangles that can be formed.

Answer: C
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Re: Set S contains nine distinct points in the coordinate plane. If exactl &nbs [#permalink] 30 Oct 2018, 18:13
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