Let z=kx+b and substitute points (1, 68) and (2, 66) to get: k+b=68 2k+b=66.

Solution: k=-2 b=70,

So z=-2x+70( 1≤x≤ 10, where x is an integer);

(2) Let the sales profit of X weeks in the first ten weeks be W (Yuan), that is, W=y(z-30).

The analytical formula of substituting y and z: y = (10x+100) (-2x+70-30) =-20x2+200x+4000.

=-20(x-5) 2 +4500，

∵-20