Mo2men wrote:Is |3m−n|+|m−2n|>|4m−3n|
(1) m>0
(2) 2n<m
Let x = 3m-n and y = m-2n, with the result that x+y = 4m-3n.
Substituting x = 3m-n, y = m-2n, and x+y = 4m-3n into |3m−n|+|m−2n|>|4m−3n|, we get:
Is
|x| + |y| > |x+y|?
Since an absolute value cannot be negative, both sides of the inequality in blue are NONNEGATIVE.
As a result, we can SQUARE THE INEQUALITY:
(|x| + |y|)² > (|x+y|)²
x² + y² + 2|x||y| > x² + y² + 2xy
2|x||y| > 2xy
|x||y| > xy.
The inequality in red will hold true if x and y have DIFFERENT SIGNS.
Since x = 3m-n and y = m-2n, question stem rephrased:
Do 3m-n and m-2n have different signs?
Statement 1: m>0
No way to determine the signs of 3m-n and m-2n.
INSUFFICIENT.
Statement 2: 2n<m
0 < m-2n
m-2n > 0.
No way to determine the sign of 3m-n.
INSUFFICIENT.
Statements combined:
Inequalities can be ADDED TOGETHER.
Adding together m>0 and m-2n > 0, we get:
m + (m-2n) > 0 + 0
2m - 2n > 0
m - n > 0.
Adding together m>0, m>0 and m-n>0, we get:
m + m + (m-n) > 0 + 0 + 0
3m-n > 0.
Since 3m-n > 0 and m-2n > 0, they have the SAME SIGN.
Thus, the answer to the question stem is NO.
SUFFICIENT.
The correct answer is
C.
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