Help me to make equation of three variables

[satwindercse]

I would like to make equation with three (x,y,z) variables to calculate the third variable from two. There is a direct relationship of third variables with other two. For example
Z= aX + bY. I want z should lie between 0 and 1. x and y both are liying between 0 and 1 both inclusive. One method is Pythagoras theorem or distance formula. which is normalised as follow:
z= 1-sqrt((1-x)^2 + (1-y)^2)/2)

some one also suggest me with z=x+y-xy .

but I also want that
Z=0 when X=0 and Y=0
Z=1 when X=1 and Y=1

Can any one help me to form the equation with any other formula? I am in dark kindly help me.

 

[Denis]

It takes 3 equations to be able to solve for 3 variables.

Please repost your question CLEARLY. Give a FULL example.

 

[satwindercse]

i have a values of two variable(X,Y) and would like to find the value of third variable(Z) form the two variables(X,Y). The conditions for the third variable are as follow:

Z=0 for X=0 and Y=0
Z=1 for X=1 and Y=1
0<=X,Y<=1

for every ( X,Y) should pass through same plane.

 

[mmm4444bot]

For example Z= aX + bY

but I also want that
Z=0 when X=0 and Y=0
Z=1 when X=1 and Y=1

Z = X/2 + Y/2