Find the answers to three high school math fill-in-the-blank questions.

5. Solution: When k=0, it does not hold.

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.