From y=y 1+y2, y=k 1x+k2/x, x= 1, y = 4；; X=2, y=5, substituted.

4=k 1+k2

5=2*k 1+k2/2

Solve this equation set to get k 1= 2 k2=2.

So y = 2x+2/x.

(2). When x=4, y=2*4+2/4= 17/2.

2. Solution: According to the meaning of the problem,

xy=3

x+y=4

X= 1, y=3, or x=3, y= 1, used to solve the equation.

So a (1, 3)

Let the analytical formula of the straight line L be y = x+b.

Substitute a (1, 3) and b=2, then L=x+2.

3. solution: substitute A (1, 2) into y=k2/x to get k2=2, that is, y = 2/x.

According to the vertical bisection of AD to OB, OB=2, that is, B (2 2,0).

Substituting B (2 2,0) and A (1, 2) into the available equation of the resolution function, we can get k=-2, b=4, that is, y=-2x+4.