$ 解:设C(a,0) $ $由题有\frac {3×|a+1|}{2}=6,解得a=3或-5$ $∴C(3,0)或(-5,0)$
$解:如图可求,AB=2+3=5,BC=\sqrt {1^{2}+3^{2}}=\sqrt {10}$ $AC=\sqrt {3^{2}+4^{2}}=5$ $∴S_{△ABC}=\frac {5×3}{2}=\frac {15}{2}$ $C_{△ABC}=5+5+\sqrt {10}=10+\sqrt {10}$ $解:由题,AO=3,BO=AB-AO=1$ $∴CO=\sqrt {2^{2}-1^{2}}=\sqrt {3}$ $∴B(1,0),C(0,\sqrt {3}),D(-4,\sqrt {3})$ $解:过D作DE⊥x轴于E,CF⊥x轴于F$ $∴S_{四边形ABCD}=S_{△ADE}+S_{四边形CDEF}+S_{△CFB}$ $=\frac {2×7}{2}+\frac {(7+5)×4}{2}+\frac {3×5}{2}$ $=38.5$
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