Given that the side length of square ABCD is 2 and point P is a point on diagonal AC, the maximum value of (. AP +. BD) ·(. Pb +. PD) is______

Given that the side length of square ABCD is 2 and point P is a point on diagonal AC, the maximum value of (. AP +. BD) ·(. Pb +. PD) is______

Take a as the coordinate origin, AB as the positive direction of X axis, ad as the positive direction of Y axis to establish a rectangular coordinate system, then a (0,0), B (2,0), C (2,2), D (0,2), ∵ P points have diagonal AC, let P (x, x), 0 < x < 2, so. AP = (x, x),. BD = (- 2,2),. Pb = (2-x, - x),. P