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y = ax - 5 y = x + 6 y = 3x + b In the xy-plane, the straight-line graphs of the three equations above each contain the point (p,r). If a and b are constants, what is the value of b?

Let's start by substituting the point (p,r) into all equations in place of (x,y) which will make step #2 a bit easier to comprehend but is not necessary to solve the question. Then, let's consider the number of variables left in each equation.

#1: r = ap - 5 (3 variables R,A,P) #2: r = p + 6 (2 variables R,P) #3: r = 3p + b (3 variables R,B,P)

1) a = 2 allows us to reduce equation #1 to the variables r and p, which are the same two variables as equation #2. Thus we have simultaneous equations. As soon as we verify that the equations are different, we know that we can solve for both variables. Once we know r and p, we can substitute in equation #3 to solve for b. Sufficient.

2) r = 17 allows us to do the same thing, more or less. It reduces equation #2 to only one variable, allowing us to solve for p. Once we have p (and r), we can use equation #3 to solve for b. Sufficient.

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Re: In the xy-plane, the straight-line graphs of the three equations above [#permalink]

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24 Sep 2015, 01:53

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if each of the 3 equations contains points (p,r) this means that they intersect in that point 1. a=2 Find the intercept Intercept for three simultaneous equations y=2x-5 y=x+6 y=3x+b Let's use the first 2 equations: plug y=x+6 in the secod equation x+6=2x-5 -> x=11, y=17 we can use the values to calulate b in the 3rd equation 17=33+b -> b=-16 SUFFICIENT

2. Here we have directly the value for Y, let's plug it in the 2nd equation y=x+6 -> 17=x+6 -> x=11, y=17; We can plug these values in the 3rd equation and find b as we did above 17=33+b -> b=-16 SUFFICIENT

Answer (D) Most important point is here to catch the hint about intersection of 3 lines at one point
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Re: In the xy-plane, the straight-line graphs of the three equations above [#permalink]

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18 Jan 2017, 15:18

we used information from both 1 and 2 then how can the answer be D... should it not be C... some one kindly clarify.......... clearly am a zero in ds and that too a big one

Re: In the xy-plane, the straight-line graphs of the three equations above [#permalink]

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18 Jan 2017, 20:18

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y = ax - 5---------1 y = x + 6---------2 y = 3x + b--------3 In the xy-plane, the straight-line graphs of the three equations above each contain the point (p,r). If a and b are constants, what is the value of b?

(1) a = 2 (2) r = 17

All three line intersect each other at common point (p,r). 1. given a = 2 putting in equation 1 .= y=2x-5 equating 1(after replacing value of a) and 2 we will get value of (p,r) putting (p,r) in equation 3 we will get value for b---suff..

2 given r = 17. putting in equation 2 we will get value of x.i'e p. Now as we know common point of intersection ,putting p,r in equation 3 , we will get value of b

In the xy-plane, the straight-line graphs of the three equations above each contain the point (p, r). If a and b are constants, what is the value of b?

1) \(a = 2\) 2) \(r = 17\)

Solution:

1) \(a = 2\) - Putting the value of a in eq1, we get: \(y = 2x - 5\) - At this point you can solve for (x, y), plug (x, y) in (eq3) and solve for (b) [though this approach might take few seconds] (alternatively, faster method) - you can skip solving for (x, y) and deduce that given 3 equations and 3 unknowns (since a is given in statement 1) we can solve for all of them (including b), since the lines are have different slopes i.e. different lines. Hence, we can get single value of 'b', proving the condition SUFFICIENT. NOTE: 3 equations and 3 unknowns does not ALWAYS mean that we can find 3 unknown. We have to make sure that 2 of them or all of them are not the same line.

2) \(r = 17\) - Since, point (p, r) lie on all the line, we can plugin the point in above equation \(r = ap - 5 => 17 = ap - 5\) \(r = p + 6 => 17 = p + 6\) \(r = 3p + b => 17 = 3p + b\) - Again, we do not need to solve for all the variables and just recognize that the above equations will lead to single value of b. Hence, SUFFICIENT.

Re: In the xy-plane, the straight-line graphs of the three equations above [#permalink]

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30 Apr 2017, 23:51

y = ax - 5 y = x + 6 y = 3x + b

In the xy-plane, the straight-line graphs of the three equations above each contain the point (p,r). If a and b are constants, what is the value of b?

1) a = 2

2) r = 17

My 2 cents.

It is important to realize from the Question stem that the 3 equations intersect as (p,r).

For 1), as we know a =2, we can equate the first and second equation to get the value of x, and then use that value of x to find value of y and the find value of b.

For 2), similarly, use r = 17 (which is value of y) to find value of x using the second equation. And then plug it back to the third equation.

Re: In the xy-plane, the straight-line graphs of the three equations above [#permalink]

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10 Sep 2017, 19:04

Would it be correct to simply say that we have 4 variables with 3 equations so eliminating any one variable gets us to three equations and three variables and is therefore sufficient? Is that logic sound?

Re: In the xy-plane, the straight-line graphs of the three equations above [#permalink]

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13 Nov 2017, 22:31

Cheryn wrote:

what is the significance of the the line "In the xy-plane, the straight-line graphs of the three equations above each contain the point (p,r)",

i solved the problem, but wud have did the same even if they didnt provide line above . as question has 3 equations with 4 variables

The highlighted statement in effect says that all these 3 lines meet each other at one point and so there is a single value of (x,y) that satisfies these 3 equations. It is only because of this highlighted statement you can solve this set of equations for a unique value of x,y, a and b.