As shown in the figure, in rectangular ABCD, ab = 30, ad = 40, P is a moving point on BC, passing through point P as PM ⊥ AC at point m, PN ⊥ BD at point n, let's ask if point P is at BC Does the value of PM + PN change during movement? If it does not change, calculate the value; if it changes, try to point out the range of change

As shown in the figure, in rectangular ABCD, ab = 30, ad = 40, P is a moving point on BC, passing through point P as PM ⊥ AC at point m, PN ⊥ BD at point n, let's ask if point P is at BC Does the value of PM + PN change during movement? If it does not change, calculate the value; if it changes, try to point out the range of change

sin∠CBD=sin∠BCA=3/5.
PM=PCsin∠BCA=PC*3/5,PN=PBsin∠CBD=PB*3/5
Therefore, PM + PN = (PC + Pb) * 3 / 5 = BC * 3 / 5 = 40 * 3 / 5 = 24 (constant value)