Official Solution:
If \(0 < 2x + 3y < 10\) and \(-10 < 3x+2y < 0\), then which of the following must be true?
I. \(x < 0\)
II. \(y < 0\)
III. \(x < y\)
A. I only
B. II only
C. I and II only
D. I and III only
E. I, II, and III
Label the inequalities as follows:
\(0 < 2x+3y < 10\) --- (1)
\(-10 < 3x+2y < 0\) --- (2)
We consider each statement individually.
Statement I:
Multiplying (1) by -2 yields \(-20 < -4x – 6y < 0\), and multiplying (2) by 3 yields \(-30 < 9x + 6y < 0\). Adding these inequalities gives \(-50 < 5x < 0\) or \(-10 < x < 0\).
This statement is true.
Statement II:
Multiplying (1) by 3 yields \(0 < 6x + 9y < 30\), and multiplying (2) by -2 yields \(0 < -6x – 4y < 20\). Adding these inequalities gives \(0 < 5y < 50\) or \(0 < y < 10\).
This statement is false.
Statement III:
Multiplying (1) by – 1 yields \(-10 < -2x – 3y < 0\). Adding this to inequality (2) yields \(-20 < x – y < 0\).
This implies that \(x < y\), and statement III is true.
Answer: D