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電子課本網(wǎng) 第87頁(yè)

第87頁(yè)

信息發(fā)布者:
解:解方程組?$\begin {cases}x + y=\dfrac {4}{3}\\x - y=\dfrac {6}{5}\end {cases}$?
兩式相加得:?$2x=\frac {4}{3}+\frac {6}{5},$?解得?$x=\frac {19}{15}。$?
兩式相減得:?$2y=\frac {4}{3}-\frac {6}{5},$?解得?$y=\frac {1}{15}。$?
?$ $?即原方程組的解為?$\begin {cases}x=\dfrac {19}{15}\\y =\dfrac {1}{15}\end {cases}。$?
?$ $?將?$\begin {cases}x + y=\dfrac {4}{3}\\x - y=\dfrac {6}{5}\end {cases}$?代入原方程組得
?$\begin {cases}\dfrac {1}{3}b-\dfrac {12}{5}a=-\dfrac {2}{15}\\2a - b=-\dfrac {4}{3}\end {cases},$?解得:?$\begin {cases}{a=\dfrac {1}{3}}\\{b = 2}\end {cases}$?
解:?$\begin {cases}x + ay = b&①\\2x + 3y = 4&②\end {cases}$?
?$ ①×2$?得?$2x + 2ay = 2b。$?
由題意知,當(dāng)?$2a = 3$?且?$2b\neq 4$?時(shí)方程組無解。
?$ $?由?$2a = 3$?解得?$a=\frac {3}{2},$?由?$2b\neq 4$?解得?$b\neq 2。$?
?$ $?所以?$a,b$?需滿足的條件是?$a=\frac {3}{2}$?且?$b\neq 2。$?
解:將方程?$2x-3ay = 4 + b$?兩邊同乘?$2$?得
?$4x-6ay = 8 + 2b。$?
?$ $?用?$4x-6ay = 8 + 2b$?與?$4x-(2a - 1)y = 18$?相減得:
?$ (-4a - 1)y=2b - 10。$?
因?yàn)榉匠探M有無數(shù)組解,則
?$-4a - 1 = 0$?且?$2b - 10 = 0。$?
所以?$a=-\frac {1}{4},$??$b = 5。$?
解:?$\begin {cases}x + 2y = 6&①\\2x - 2y+mx = 8&②\end {cases}$?
① + ②得:?$x + 2y+2x - 2y+mx = 6 + 8,$?
即?$(3 + m)x = 14,$?所以?$x=\frac {14}{3 + m}。$?
?$ $?由?$①$?得?$y = 3-\frac {x}{2}。$?
因?yàn)榉匠探M有整數(shù)解,且?$m $?是整數(shù),?$x$?是偶數(shù),
所以?$3 + m=\pm 1$?或?$3 + m=\pm 7$?
?$ $?當(dāng)?$3 + m = 1$?時(shí),?$m=-2,$?此時(shí)
?$x = 14,$??$y = 3 - 7=-4;$?
?$ $?當(dāng)?$3 + m=-1$?時(shí),?$m=-4,$?此時(shí)
?$x=-14,$??$y = 3 + 7 = 10;$?
?$ $?當(dāng)?$3 + m = 7$?時(shí),?$m = 4,$?此時(shí)
?$x = 2,$??$y = 3 - 1 = 2;$?
?$ $?當(dāng)?$3 + m=-7$?時(shí),?$m=-10,$?此時(shí)
?$x=-2,$??$y = 3 + 1 = 4;$?
綜上,整數(shù)?$m $?的值為?$-2$?或?$-4$?或?$-10$?或?$4。$?