|x-7| = 2x + 1
點解要分case做? (e.g. for x>7 & x<7)
點解唔可以
x-7 = 2x+1 or -(2x +1) ?
ABSOLUTE VALUE (20點 ! !)
2008-07-29 1:10 am
回答 (3)
2008-07-29 6:20 am
✔ 最佳答案
可以,不過當你睇 x - 7 = 2x + 1 or -(2x + 1)既時候,你做出黎既答案sub返入最原先條式度既時候,左右邊有機會差左一個正負號.個原因係,我地其實知道 |x - 7|幾時等於2x + 1,幾時等於 -(2x+1)既. 因為|x-7| 一定要係正數,所以我地知道佢係無可能等於一個負數,咁即係話 2x + 1大過0既時候,|x-7|先可以等於佢. 唔係既話就要等於 -(2x+1).
所以,個analysis其實同分case一樣,不過諗左先分case,定係分左case先諗. 又或者甚至計晒之後,先check個答案係唔係valid answer都得.
2008-08-04 2:56 am
可以x-7 = 2x+1 or -(2x +1)咁做!
不過你要注意2x+1>0
即x>-1/2
解:
x-7 = 2x+1 or x-7= -(2x +1)
x=-8(reject) or 3x=6
∴x=2
不過你要注意2x+1>0
即x>-1/2
解:
x-7 = 2x+1 or x-7= -(2x +1)
x=-8(reject) or 3x=6
∴x=2
2008-07-29 1:41 am
Both methods are acceptable.
2008-07-28 18:08:32 補充：
But make sure to check the answers if they satisfy the original equation, especially the second method because it did not put down any condition at the beginning.
2008-07-28 18:08:32 補充：
But make sure to check the answers if they satisfy the original equation, especially the second method because it did not put down any condition at the beginning.
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