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Bunuel
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Bunuel
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I think this is a high-quality question and I agree with explanation.
I agree with this
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hey Bunuel your way is the easiest to work with however it didn't strike me while taking the CAT. I did as follows:
X intercept of a line y=mx+c = -c/m
m of ax+by+c=0 is -a/b (substituting in the above)
x intercept. = cb/a
now to find whether cb/a <0 we will need both statements.
Could you please help me with where I am going wrong? Thanks!!
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hey Bunuel your way is the easiest to work with however it didn't strike me while taking the CAT. I did as follows:
X intercept of a line y=mx+c = -c/m
m of ax+by+c=0 is -a/b (substituting in the above)
x intercept. = cb/a
now to find whether cb/a <0 we will need both statements.
Could you please help me with where I am going wrong? Thanks!!


ax + by + c = 0

by = -ax - c

y = -a/b*x - c/b

x-intercept = (c/b)/(-a/b) = c/b*(-b/a) = -c/a. The same as in the solution above.
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Bunuel
Official Solution:


If \(abc \ne 0\), is the x-intercept of the line \(ax + by + c = 0\) negative?

The x-intercept of a line is the value of \(x\) when \(y=0\). For the line \(ax+by+c=0\), the x-intercept is given by \(x=-\frac{c}{a}\). The question essentially asks: Is \(-\frac{c}{a} < 0\)? Which can be rephrased as: Is \(\frac{c}{a} > 0\)? This boils down to whether \(c\) and \(a\) have the same sign.

(1) \(ba \lt 0\).

This is not sufficient as it doesn't tell us if \(c\) and \(a\) have the same sign.

(2) \(ac \gt 0\).

From this statement, it's clear that \(c\) and \(a\) have the same sign. Hence, it's sufficient.


Answer: B


if b is +ve => a -ve and c -ve (therefore x intercept is positive),
if b is -ve => a is +ve and c is +ve (therefore x intercept is negative)

this is how i concluded that this statements are not sufficient together, could you please pointout my logical fallacy ehre since i am a little confused.
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Bunuel
Official Solution:


If \(abc \ne 0\), is the x-intercept of the line \(ax + by + c = 0\) negative?

The x-intercept of a line is the value of \(x\) when \(y=0\). For the line \(ax+by+c=0\), the x-intercept is given by \(x=-\frac{c}{a}\). The question essentially asks: Is \(-\frac{c}{a} < 0\)? Which can be rephrased as: Is \(\frac{c}{a} > 0\)? This boils down to whether \(c\) and \(a\) have the same sign.

(1) \(ba \lt 0\).

This is not sufficient as it doesn't tell us if \(c\) and \(a\) have the same sign.

(2) \(ac \gt 0\).

From this statement, it's clear that \(c\) and \(a\) have the same sign. Hence, it's sufficient.


Answer: B


if b is +ve => a -ve and c -ve (therefore x intercept is positive),
if b is -ve => a is +ve and c is +ve (therefore x intercept is negative)

this is how i concluded that this statements are not sufficient together, could you please pointout my logical fallacy ehre since i am a little confused.

The red part is not correct. You can substitute numbers there and check to see that you'd still get a negative x-intercept.
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I've revised the question and solution, incorporating additional details for improved clarity. I trust this makes it more comprehensible.
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