1)|OA|=|OB|=2,|OC|=17=|OA+OC|^2=|OA|^2+|OC|^2+2OA*OC=4+1-2*2cosa=5-4cosa∴cosa=-1/2, a=120OB和OC夹角t有cost=OB*OC/|OB||OC|=2sina/2=sin120=sin60=cos30∴OB和OC夹角为302)AC=(cosa-2,sina),BC=(cosa,sina-2)AC*BC=cosa(cosa-2)+sina(sina-2)=1-2cosa-2sina=0∴sina+cosa=√2sin(a-π/4)=1/2, sin(a-π/4)=√2/4<√2/2, ∵0<α<π, -π/4<α-π/4<3π/4,∴-π/4<a-π/4<π/4, c...
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