Re: [計量] 幾題數學~~
※ 引述《MyGod (請不要企圖用此ID!!)》之銘言：
: ※ 引述《butini (hackneyed)》之銘言：
: : Test 8
: : 6.x+y+z<0 z>x+y+1
: : Column A: z
: : Column B:1
: : Ans:D
: : *有判斷的好方法嗎
: ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
: x+y+z<0 ==>-z>x+y----(1)
: 並且 z>x+y+1-----(2)
: (2)-(1)得: 2z>1 ==>z>1/2跟1比.Z可以比1小.可以等於1.也可以比1大.so.無法判
: 斷.選D
不等式碰到減要小心喔~
-3>-5 (1)
3> 2 (2) 依上面的算法(2)-(1)得 6>7
原因在>乘上-1就變<了
原題目是z和1比較，所以要看x+y是否大於等於0
但x+y<-1/2 ---(1)+(2) 故無法判斷
若x+y>=0，則 z>(x+y)+1>=(0)+1=1 即 z>1
若x+y<0，則z > 小於1的數 ，即z的最小值可以小於等於1，故無法判斷
這兩式圍出三維空間的一塊區域(感覺就像用兩刀切了四分之一塊)
只要x+y<-1/2
z就有一定的範圍
而不論z是多少，都有x,y符合
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◆ From: 218.164.49.106
※ 編輯: JieJuen 來自: 218.164.49.106 (06/17 20:07)
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