中二數學(~急~)

x = 7+ y __ (1)
sub (1) into x - 9 = 2 (y - 9)
7 + y - 9 = 2 (y - 9)
2 + y = 2y - 18
y = 20
x = 7 +20 = 27

2007-10-04 21:39:26 補充：
x = 7+ y __ (1)sub (1) into x - 9 = 2 (y - 9)7 + y - 9 = 2 (y - 9)- 2 + y = 2y - 18y = 16x = 7 + 16 = 23

先將 {x-y=7 變成 x=7+y
之後代入第2條式...> x-9=2(y-9) 寫做 7+y-9=2(y-9)
先乘右邊的數...> 7+y-9=2y-18
再放x及y放好...> 7-9+18=2y-y
之後就可以計到...> y=?
代y入x=7+y ...> x=7+?
就可以計到...自己計la...我唔知我會唔會寫錯

{x-y=7-----------(1)

{x-9=2(y-9)------------(2)

from (1) x=7+y ----------------(3)

sub (3) into (2)
(7+y)-9=2(y-9)
7+y -9 =2y -18
7-9+18 =2y- y
16 = y ---------(4)

sub (4) into (1)
x-16 =7
x =23



最完整答案

x-y=7----- *
x-9=2(y-9) ----- @

from *
x-y=7
x= 7+y ----- #

form @
x-9=2(y-9)
x - 9 = 2y -18
x- 2y +9 =0 ----$

Sub # into $
(7+y)-2y+9=0
7+9-y = 0
2-y = 0
y = 2

sub y = 2 into *
x -y =7
x - 2 =7
x = 9

{x-y=7--------------1

{x-9=2(y-9)----------2

由 1 :x=y+7 代入2
y+7-9=2y-18
y=16
x=23

x-y = 7 ..............1
x-9 = 2(y-9).........2
from 1,
y=x-7
put y = x-7 in 2
x-9 = 2(x-7-9)
x-9 = 2x-32
x = 23
y = x-7=23-7=16