∵cosa=-√5/5 180°
tan(a-b)=(tana-tanb)/(1+tana*tanb)
=(1/2-1/3)/[1+(1/2)*(1/3)]=1/7
∴a-b=Arctan1/7
cosa=-√5/5,π<α<3π/2
sina=-2√5/5
tanB=1/3,,0
cosB=3√10/10
0
π/2<α-B<3π/2
sin(α-B)
=sinacosB-cosasinB
=-2√5/5*3√10/10-(-√5/5)*√10/10
=-6√50/50+√50/50
=-5√50/50
=-√50/10
=-5√2/10
=-√2/2
π/2<α-B<3π/2
π<α-B<3π/2
所以sin(α-B)=-√2/2
即α-B=5π/4
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