A. ac<0
B. ac>0
C. b^2 -4ac<0
D. b^2 -4ac=0
Express why the answer is A.
更新1:
Explain why the answer is A.
quadratic equations questions...唔該幫幫我.(2)
2008-09-26 6:39 am
**11. The quadratic equation ax^2+bx+c=0 MUST have two unequal roots when
A. ac<0
B. ac>0
C. b^2 -4ac<0
D. b^2 -4ac=0
Express why the answer is A.
A. ac<0
B. ac>0
C. b^2 -4ac<0
D. b^2 -4ac=0
Express why the answer is A.
回答 (2)
2008-09-26 3:17 pm
✔ 最佳答案
For the equation to have unequal roots, delta must be positive, that is b^2 - 4ac must be positive, b^2 is certainly positive, so if ac is negative, -4ac is also certainly positive, so delta is certainly positive, thus ac< 0 is the answer. 
2008-09-26 4:57 pm
ax^2 bx c=0 MUST have two unequal roots when
b^2 -4ac>0
b^2 >4ac
As b^2 > 0
¯¯
When ac<0, b^2 >4ac MUST hold.
So,
when ac<0, ax^2 bx c=0 MUST have two unequal roots.
b^2 -4ac>0
b^2 >4ac
As b^2 > 0
¯¯
When ac<0, b^2 >4ac MUST hold.
So,
when ac<0, ax^2 bx c=0 MUST have two unequal roots.
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