已知方程组5x+7y=5m+4和2x-y=3-m的解x,y的和不小于4,求m的范围.（要求有过程）
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5x-7y=5m+4 -> 10x-14y=10m+8 (1)
2x-y=3-m -> 10x-5y=15-5m (2)
(2)-(1):9y=7-15m -> y=(7-15m)/9 (3)
5x-7y=5m+4 (4)
2x-y=3-m -> 14x-7y=21-7m (5)
(5)-(4):9x=17-12m -> x= (17-12m)/9 (6)
(3)+(6):x+y=(17-12m)/9+(7-15m)/9=(24-27m)/9=(8-9m)/3
x+y>=4 -> (8-9m)/3>=4 -> 8-9m>=12 -> 9m m
2x-y=3-m -> 10x-5y=15-5m (2)
(2)-(1):9y=7-15m -> y=(7-15m)/9 (3)
5x-7y=5m+4 (4)
2x-y=3-m -> 14x-7y=21-7m (5)
(5)-(4):9x=17-12m -> x= (17-12m)/9 (6)
(3)+(6):x+y=(17-12m)/9+(7-15m)/9=(24-27m)/9=(8-9m)/3
x+y>=4 -> (8-9m)/3>=4 -> 8-9m>=12 -> 9m m
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