Substitute x=t and y=0 into y=x2+bx to get t2+bt=0.

∫t > 0，

∴b=﹣t；

(2)① unchanged.

As shown in fig. 6, when x= 1 and y = 1-t, then M( 1, 1-t),

∫tan∠AMP = 1，

∴∠amp=45；

②S=S quadrilateral amnp-s △ PAM = s △ dpn+s trapezoid ndam-s △ PAM = (t-4) (4t-16)+[(4t-16)).

The solution t2-t+6 =,

Get: t 1=，t2=，

∫4 < t < 5，

∴t 1= Give up,

∴t=。

(3)