quadratic equations questions...唔該幫幫我.(2)

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.

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.