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In the xy-plane

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bhumika.k.shah Legendary Member Default Avatar
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In the xy-plane

Post Thu Jan 07, 2010 9:45 pm
In the xy-plane, what is the slope of the line with equation 3x + 7y = 9?
A. - 7/3
B. - 3/7
C. 3/7
D. 3
E. 7

Ans : B

is it true? if so , why so ??

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GMAT/MBA Expert

Post Thu May 21, 2015 1:23 pm
bhumika.k.shah wrote:
In the xy-plane, what is the slope of the line with equation 3x + 7y = 9?
A. - 7/3
B. - 3/7
C. 3/7
D. 3
E. 7
Rewriting the equation in slope y-intercept form is a nice fast approach.

Alternatively, you can find any two points that are on the line and then use the slope formula.
Slope of line connecting the points (x1, y1) and (x2, y2) = (y2 - y1)/(x2-x1)

So, let's find 2 points that are on the line.
In other words, we're looking for COORDINATES x and y such that 3x + 7y = 9

To find one point, let's let x = 0 and solve for y.
We get: 3(0) + 7y = 9
So, 7y = 9, which means y = 9/7
So, one point on the line is (0, 9/7)

To find another point, let y = 0 and solve for x.
We get: 3x + 7(0) = 9
So, 3x = 9, which means x = 3
So, another point on the line is (3, 0)

Applying the slope formula, we get SLOPE = (0 - 9/7)/(3 - 0)
= (-9/7)/3
= (-9/7)(1/3)
= -3/7

Answer: B

Cheers,
Brent

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Post Thu May 21, 2015 10:20 am
bhumika.k.shah wrote:
In the xy-plane, what is the slope of the line with equation 3x + 7y = 9?
A. - 7/3
B. - 3/7
C. 3/7
D. 3
E. 7

Ans : B

is it true? if so , why so ??
Solution:

The easiest way to determine the slope of the line is to put the equation of the line into slope-intercept form, which is y = mx + b, where m = slope and b = the y-intercept. We must rewrite the given equation 3x + 7y = 9 to get y by itself on the left.

3x + 7y = 9

7y = -3x + 9

y = (-3/7)x + 9/7

Using the general slope-intercept form of y = mx + b, we see that the slope m = -3/7

The answer is B

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2009wish Junior | Next Rank: 30 Posts Default Avatar
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Post Thu Jan 07, 2010 9:59 pm
Equation for the slope of a line is .... y=mx +c

when the equaiton is rewritten .....y=-3/7x+9/7

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bhumika.k.shah Legendary Member Default Avatar
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Post Thu Jan 07, 2010 10:10 pm
2009wish wrote:
Equation for the slope of a line is .... y=mx +c

when the equaiton is rewritten .....y=-3/7x+9/7
Why the - sign ? why -3/7 ? and why not - 9/7 then ?

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papgust Community Manager
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Post Thu Jan 07, 2010 10:23 pm
Let's re-phrase the eqn 3x + 7y = 9 to the form
y= mx + c

7y = -3x + 9
y = (-3/7)x + 9/7

m is the slope of line. So, Slope is -3/7.

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