$ (\frac {5}{4},0),(-\sqrt {5},0),(\sqrt {5},0),(4,0)$
$ 解:(1)由題,C(0,2),D(4,2)$ $S_{四邊形ABCD}=4×2=8$ $(2)設(shè)P(a,0)或P(0,b)$ $由題有,\frac {1}{2}×2×|a+1|=8,解得a=7或-9\ \ ∴P(7,0)或P(-9,0)$ $\frac {1}{2}×1×|b-2|=8,解得b=18或-14\ \ ∴P(0,18)或P(0,-14)$ $綜上,P(7,0)或(-9,0)或(0,18)或(0,-14)$
$解:(3)S_{△ABC}=3×5-\frac {1}{2}×(1×2+2×5+3×3)=4.5$ (2)解:∵∠AOB=90°-30°=60° ∴△AOB為等邊三角形 ∴AB=OB=4 即A,B兩點間距離為4
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