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# O-Level Additional Mathematics. Helps needed! watch

1. Find the values of k for which the quadratic equation 9x^2 - kx + (k-7) =0 has one positive and one negative root.
2. (Original post by ming jian)
Find the values of k for which the quadratic equation 9x^2 - kx + (k-7) =0 has one positive and one negative root.
Let the 2 roots be and
then the equation

from this the original equation is

so for k

because one of the root is positive the other is negative
Another limit for k is that there should be two real roots
that is

but for k-7<0 this equation is always true.
From this follows ...... ???
3. agree with ztibor

in general the product of roots is c/a which in this case is ( k-7 )/9 and this must be negative

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Updated: March 14, 2013
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