Quote:
Is the slope of the line that passes through the point (x, y) negative?
(1) The x-intercept of the line is a such that a < x
(2) Both x and y are less than 0
AnujL
egmatplease provide a detailed explanation
Hey
AnujL,
Thanks for your query.
CONCEPT RECAP: - Slope of a line (used in question stem): To find the slope of any line, we need the coordinates of two points on that line.
- If (x, y) and (a, b) are the two points on a line, then slope of that line = \(\frac{(y−b)}{(x−a)}\)
- x-intercept of a line (used in statement 1): The x-intercept of a line is the point where the line cuts the x-axis. At this point, the y-coordinate is 0.
SOLUTION: What should be the first thing that we do once we see a DS question? - Analyse the question stem completely!
Question stem analysis:All we are given in the question stem is a line that passes through point (x, y). We need to find whether the slope of this line is negative.
From Concept Recap - Point 1, we know that to find the slope of any line, we need two points on that line. And here we only have
one point, (x, y). Since we can’t go further with this, let’s analyze the statements one by one.
Statement 1: “The x-intercept of the line is ‘a’ such that a < x”.
- Using Concept Recap – Point 2 along with statement 1, we can infer that point (a, 0) lies on the line. (See how we just got another point on the line!)
- Okay! So, now we have two points on the line - everything we need to calculate its slope.
- Slope =\(\frac{(y−0)}{(x−a)}\) = \(\frac{(y)}{(x−a)}\)
- Now, from statement 1, we also know that a < x.
- Thus, (x – a) > 0 (the denominator of the slope is positive).
- So, whether the slope is negative will depend on the value of ‘y’.
- If y > 0, the slope will be positive,
- and if y < 0, the slope will be negative.
So, even after knowing the two points, we still cannot tell whether the slope is negative. Thus, statement 1 alone is
INSUFFICIENT and options A and D are rejected!
Statement 2: “Both x and y are less than 0”.
This information just tells us more about the same point, (x, y) that we have from the stem. But we cannot find the slope without one more point, right?
And therefore, statement 2 alone is
INSUFFICIENT, and thus, option B is rejected, too!
IMPORTANT: Make sure you do not drag anything from statement 1 while analyzing statement 2. It is a common error and will surely take you to the wrong choice!Statements 1 and 2 together: Finally, since each statement alone is insufficient, let’s combine our conclusions from both the statements.
- From statement 1, we inferred that the slope = \(\frac{(y)}{(x−a)}\), and (x – a) > 0.
- From statement 2, we got that y < 0.
Recall that we only needed one more thing after statement 1 (the sign of y) to determine the sign of the slope. And then statement 2 gave us just that. We can conclude here itself, therefore, that the statements together are sufficient and thus, the answer is choice C.
But, just to take the Math through, let’s actually combine and see what we get for the slope:
- Slope = \(\frac{(y)}{(x−a)}\)=\(\frac{(−)}{(+)}\) = (-)ve
We, thus, get a SURE YES to the question asked using
both statements together.
Correct Answer:
Option C Hope this helps!
Best,
Aditi Gupta
Quant Expert,
e-GMAT