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電子課本網(wǎng) 第77頁

第77頁

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 解:?$2(x - 2)-5(x + 4)>- 30$?
?$2x - 4 - 5x - 20>- 30$?
?$2x - 5x>- 30 + 4 + 20$?
?$-3x>- 6$?
?$x<2$?
在數(shù)軸上表示如圖所示

解:?$6x - 3(x + 2)>2(2x - 5)-6$?
?$6x - 3x - 6>4x - 10 - 6$?
?$6x - 3x - 4x>- 10 - 6 + 6$?
?$-x>- 10$?
?$x<10$?
在數(shù)軸上表示如圖所示

解:?$3x + 3 - 1<4x - 4 + 3$?
?$3x - 4x<- 4 + 3 - 3 + 1$?
?$-x<- 3$?
?$x>3$?
∴最小整數(shù)解為?$4$?
?$ $?將?$x = 4$?代入?$mx - nx = 6,$?得?$4\ \mathrm {m} - 4n = 6,$?即?$m - n=\frac 32$?
∴?$m - n - 3=\frac 32-3=-\frac 32$?
解:?$(1)4y - 2y = - 2\ \mathrm {m} + 5 - 1$?
?$2y = - 2\ \mathrm {m} + 4$?
?$y = - m + 2$?
∵方程的解是負數(shù),∴?$-m + 2<0,$??$m>2$?
?$ (2)$?∵?$m>2,$?∴?$m $?取最小整數(shù)值為?$3$?
?$x - 2>3x + 1$?
?$2x - 3x>1 + 2$?
?$-x>3$?
?$x<- 3$?
解:解不等式?$1+\frac {3x}{m}>\frac {x}{m}+\frac 9{m}$?
?$ ①$?當?$m > 0$?時,去分母得:?$m + 3x > x + 9$?
移項得:?$3x - x > 9 - m$?
合并同類項得:?$2x > 9 - m$?
?$ $?系數(shù)化為?$1$?得:?$x >\frac 12(9 - m)$?
?$ $?解不等式?$x + 1 >\frac {x - 2 + m}3$?
去分母得:?$3x + 3 > x - 2 + m$?
移項得:?$3x - x > m - 2 - 3$?
合并同類項得:?$2x > m - 5$?
?$ $?系數(shù)化為?$1$?得:?$x >\frac {m - 5}2$?
?$ $?當?$\frac 12(9 - m)=\frac {m - 5}2$?時,解得?$m = 7$?
∴存在整數(shù)?$m = 7$?使關于?$x$?的兩個不等式解集相同
?$ ②$?當?$m < 0$?時,去分母得:?$m + 3x < x + 9$?
移項得:?$3x - x < 9 - m$?
合并同類項得:?$2x < 9 - m$?
?$ $?系數(shù)化為?$1$?得:?$x <\frac 12(9 - m)$?
∵當?$m < 0$?時,?$\frac {m - 5}2<0,$??$\frac 12(9 - m)>0,$?而?$x >\frac {m - 5}2$?與?$x <\frac 12(9 - m)$?的不等號方向是相反
∴當?$m < 0$?時解集不相同
綜上,當整數(shù)?$m = 7$?時,不等式?$1+\frac {3x}{m}>\frac {x}{m}+\frac 9{m}$?與?$x + 1 >\frac {x - 2 + m}3$?的解集相同,解集是?$x > 1$?