We are given a system of two linear equations with two variables $x$ and $y$, and a parameter $m$: $(m+1)x - my = 4$ $3x - 5y = m$ We want to find the values of $m$ for which the system has a solution $(x, y)$ such that $x - y < 2$.

AlgebraLinear EquationsSystems of EquationsInequalitiesParameterSolution Sets
2025/5/26

1. Problem Description

We are given a system of two linear equations with two variables xx and yy, and a parameter mm:
(m+1)xmy=4(m+1)x - my = 4
3x5y=m3x - 5y = m
We want to find the values of mm for which the system has a solution (x,y)(x, y) such that xy<2x - y < 2.

2. Solution Steps

First, we solve the system of equations for xx and yy in terms of mm.
From the second equation, we can express xx in terms of yy and mm:
3x=5y+m3x = 5y + m, so x=5y+m3x = \frac{5y + m}{3}.
Substitute this expression for xx into the first equation:
(m+1)(5y+m3)my=4(m+1)(\frac{5y + m}{3}) - my = 4
(m+1)(5y+m)3my=4\frac{(m+1)(5y + m)}{3} - my = 4
(m+1)(5y+m)3my=12(m+1)(5y + m) - 3my = 12
5my+m2+5y+m3my=125my + m^2 + 5y + m - 3my = 12
2my+5y+m2+m12=02my + 5y + m^2 + m - 12 = 0
y(2m+5)=12mm2y(2m + 5) = 12 - m - m^2
If 2m+502m + 5 \neq 0, then y=12mm22m+5y = \frac{12 - m - m^2}{2m + 5}.
Substituting this into the expression for xx:
x=5y+m3=5(12mm22m+5)+m3=5(12mm2)+m(2m+5)3(2m+5)=605m5m2+2m2+5m3(2m+5)=603m23(2m+5)=20m22m+5x = \frac{5y + m}{3} = \frac{5(\frac{12 - m - m^2}{2m + 5}) + m}{3} = \frac{5(12 - m - m^2) + m(2m + 5)}{3(2m + 5)} = \frac{60 - 5m - 5m^2 + 2m^2 + 5m}{3(2m + 5)} = \frac{60 - 3m^2}{3(2m + 5)} = \frac{20 - m^2}{2m + 5}.
Now, we have x=20m22m+5x = \frac{20 - m^2}{2m + 5} and y=12mm22m+5y = \frac{12 - m - m^2}{2m + 5}.
We want xy<2x - y < 2, so 20m22m+512mm22m+5<2\frac{20 - m^2}{2m + 5} - \frac{12 - m - m^2}{2m + 5} < 2.
20m2(12mm2)2m+5<2\frac{20 - m^2 - (12 - m - m^2)}{2m + 5} < 2
20m212+m+m22m+5<2\frac{20 - m^2 - 12 + m + m^2}{2m + 5} < 2
8+m2m+5<2\frac{8 + m}{2m + 5} < 2
8+m2m+52<0\frac{8 + m}{2m + 5} - 2 < 0
8+m2(2m+5)2m+5<0\frac{8 + m - 2(2m + 5)}{2m + 5} < 0
8+m4m102m+5<0\frac{8 + m - 4m - 10}{2m + 5} < 0
3m22m+5<0\frac{-3m - 2}{2m + 5} < 0
3m+22m+5>0\frac{3m + 2}{2m + 5} > 0
The critical points are m=23m = -\frac{2}{3} and m=52m = -\frac{5}{2}.
We test intervals:

1. $m < -\frac{5}{2}$: $3m + 2 < 0$ and $2m + 5 < 0$, so $\frac{3m + 2}{2m + 5} > 0$.

2. $-\frac{5}{2} < m < -\frac{2}{3}$: $3m + 2 < 0$ and $2m + 5 > 0$, so $\frac{3m + 2}{2m + 5} < 0$.

3. $m > -\frac{2}{3}$: $3m + 2 > 0$ and $2m + 5 > 0$, so $\frac{3m + 2}{2m + 5} > 0$.

Thus, the solution is m<52m < -\frac{5}{2} or m>23m > -\frac{2}{3}.
Also, we assumed 2m+502m + 5 \neq 0, so m52m \neq -\frac{5}{2}.

3. Final Answer

m(,52)(23,)m \in (-\infty, -\frac{5}{2}) \cup (-\frac{2}{3}, \infty)

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