我想問係店樣去決定一條數既 D 係D>0 or D < 0
我舉個例:
find the range of values of k if x^2 - 4x + k > 0
點決定個 D 係>0 or <0?
有咩方法可以睇得出
A maths (inequalities)
2008-09-30 3:29 am
回答 (2)
2008-10-05 8:37 pm
其實好簡單, x^2 - 4x + k > 0 係咩意思呢?
x^2 - 4x + k > 0 即係話 x^2 - 4x + k 係 x-axis 以上 , 錦既然係x-axis 以上, 錦又點會有交點呢?
所以D <0
錦x^2 - 4x + k > 0 又會點呢? 好簡單 , D <0
所以我地可以得出:
f(x)=ax^2+bx+c
f(x)<0 <=> D>0
f(x)>0 <=> D<0
2008-10-05 12:38:13 補充：
all a>0
x^2 - 4x + k > 0 即係話 x^2 - 4x + k 係 x-axis 以上 , 錦既然係x-axis 以上, 錦又點會有交點呢?
所以D <0
錦x^2 - 4x + k > 0 又會點呢? 好簡單 , D <0
所以我地可以得出:
f(x)=ax^2+bx+c
f(x)<0 <=> D>0
f(x)>0 <=> D<0
2008-10-05 12:38:13 補充：
all a>0
2008-09-30 4:02 am
For a quadratic function y to be greater than zero for all values of x, that means the curve will never touch or intersect the x -axis. For the curve never touch or intersect the x - axis, delta must be < 0.
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原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20080929000051KK01655