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. One method is Pythagoras theorem or distance formula. Can any one help me to form the equation with any other formula? I am in dark kindly help me.