According to the related properties of quadratic function, it is necessary to do kx? -(k-2) x+k < 0 holds.
So: k > 0 and △=(k-2)? -4k? 0 and (k+2) (3k-2) > 0.
Therefore: k > 2/3
6. Because 1/x+9/y= 1.
Therefore: x+y = (x+y) (1/x+9/y) =10+y/x+9x/y ≥10+6 =16.
Therefore, when x=4 and y= 12, the minimum value of x+y is 16.
7. because X > 4, that is, x-4 > o.
Therefore: x-4+ 1/(x-4) ≥2.
Therefore: 4-x+ 1/(4-x)≤-2.
Therefore: y =-x+1(4-x) = 4-x+1/(4-x)-4 ≤-6.
Therefore, the maximum value of y is -6, and x-4= 1/(x-4), that is, x=5.