a: 
b: PTHĐGĐ là:
-x^2-2x+3=0
=>x^2+2x-3=0
=>(x+3)(x-1)=0
=>A(-3;-9); B(1;-1)
\(OA=\sqrt{\left(-3\right)^2+\left(-9\right)^2}=3\sqrt{10}\)
\(OB=\sqrt{1^2+\left(-1\right)^2}=\sqrt{2}\)
\(AB=\sqrt{\left(1+3\right)^2+\left(-1+9\right)^2}=4\sqrt{5}\)
\(cosO=\dfrac{90+2-80}{2\cdot3\sqrt{20}}=\dfrac{\sqrt{5}}{5}\)
=>\(sinO=\dfrac{2\sqrt{5}}{5}\)
\(S_{OAB}=\dfrac{1}{2}\cdot\dfrac{2\sqrt{5}}{5}\cdot3\sqrt{20}=6\)
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