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Re: If a and b are positive numbers, what are the coordinates of [#permalink]

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09 Sep 2007, 17:39

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b14kumar wrote:

If a and b are positive numbers, what are the coordinates of the midpoint of line segment CD in xy plane?

1. The coordinates of C are (a,1-b) 2. The coordinates of D are (1-a,b)

- Brajesh

to find the mid point u just take the mean of x1 and x2 and mean of y1 and y2, that leaves us with mid point coordinates of x=1/2 y=1/2
to have it we need to use both statements
C

If a and b are positive numbers, what are the coordinates pf the midpoint of line segment CD in the xy plane?

1) C (a, 1 - b)

2) D (1 - a, b)

OA = C

Coordinates of the midpoint \(M (x_m,y_m)\) of the line segment AB, (\(A (x_1,y_1)\) and \(B (x_2,y_2)\)) are \(x_m=\frac{x_1+x_2}{2}\) and \(y_m=\frac{y_1+y_2}{2}\)

So we find the coordinates of the midpoint in terms of a and b right?

Thanks

If it were so, then answer would be E. In DS questions statement(s) is (are) sufficient only if we can get single numerical value of variable, midpoint coordinates, etc.
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Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.

If a and b are positive numbers, what are the coordinates of the midpoint of line segment CD in the xy-plane?

(1) The coordinates of C are (a, 1-b) (2) The coordinates of D are (1-a, b)

There are 2 variables (x,y) and 2 equations are given by the 2 conditions, so there is high chance (C) will be the answer. Looking at the conditions together, midpoint=(a+1-a/2, 1-b+b/2)=(1/2,1/2) This is unique and sufficient, so the answer becomes (C).

For cases where we need 2 more equations, such as original conditions with “2 variables”, or “3 variables and 1 equation”, or “4 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 70% chance that C is the answer, while E has 25% chance. These two are the majority. In case of common mistake type 3,4, the answer may be from A, B or D but there is only 5% chance. Since C is most likely to be the answer using 1) and 2) separately according to DS definition (It saves us time). Obviously there may be cases where the answer is A, B, D or E.
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Re: If a and b are positive numbers, what are the coordinates of [#permalink]

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17 Dec 2015, 10:40

In order to find the midpoint of a line segment, both the end points of a line are needed. What further solves the question is the victory over laziness before one selects E due to presence of variables from the two statements; they get cancelled and the midpoint is (0.5,0.5)

Also, I applied a learning from a previous question. Positive numbers and Positive Integers are different. While we tend to know the difference, remembering the same every time we see integers and numbers pop up on the screen is important. Be careful folks!
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If a and b are positive numbers, what are the coordinates of [#permalink]

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04 Aug 2016, 16:48

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Bunuel wrote:

In DS questions statement(s) is (are) sufficient only if we can get single numerical value of variable, midpoint coordinates, etc.

I know what you mean (solving in terms of a variable is rarely sufficient), but we have to be careful--this is not always true. If the question being asked is a yes/no question ("is x positive?" or "is y less than 0?"), then the statements could be sufficient even if we can get multiple values for x or y, so long as they always result in the same answer to the question being asked (either "always yes", or "always no").

There are some cases on DS where both solving in terms of a variable, and getting multiple numerical values, can be sufficient.

For example, if the question asks, "is x > 5?" and statement #1 leads me to the conclusion that x = 10, 15, or 20, then I can conclude "sufficient," even though I found multiple solutions for x, because the answer to the question being asked is always yes.

This can also be true when solving in terms of a variable. If the question asked "is x positive?" and I am able to solve for x as \((y^2+1)\), then I know that regardless of the value of y, x must be positive. Hence it would be sufficient.
_________________

Harvard grad and 770 GMAT scorer, offering high-quality private GMAT tutoring, both in-person and via Skype, since 2002.

In DS questions statement(s) is (are) sufficient only if we can get single numerical value of variable, midpoint coordinates, etc.

I know what you mean (solving in terms of a variable is rarely sufficient), but we have to be careful--this is not always true. If the question being asked is a yes/no question ("is x positive?" or "is y less than 0?"), then the statements could be sufficient even if we can get multiple values for x or y, so long as they always result in the same answer to the question being asked (either "always yes", or "always no").

There are some cases on DS where both solving in terms of a variable, and getting multiple numerical values, can be sufficient.

For example, if the question asks, "is x > 5?" and statement #1 leads me to the conclusion that x = 10, 15, or 20, then I can conclude "sufficient," even though I found multiple solutions for x, because the answer to the question being asked is always yes.

This can also be true when solving in terms of a variable. If the question asked "is x positive?" and I am able to solve for x as \((y^2+1)\), then I know that regardless of the value of y, x must be positive. Hence it would be sufficient.

Yes. This is a value question so I think it's clear what I meant: When a DS question asks about the value of some variable, then the statement(s) is sufficient ONLY if you can get the single numerical value of this variable.
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Re: If a and b are positive numbers, what are the coordinates of [#permalink]

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18 Sep 2017, 01:16

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