当Q在AC上
因为AC = 3,BC = 4
所以AB = 5
又AP/AQ = AC/AB
所以AQ = 5X/3
S = 5/6*X^2
当Q在BC上
CP = 5-X
QP = 3/4*(5-X)
S = -3/8*X^2+15/8*X