三角形ABC中,C=2B,三边记为a,b,c,则sin3B/sinB等于( )A.b/aB.a/bC.c/aD.a/c请写出步骤.谢谢!

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选B。A=180-C-B=180-3B，sinA=sin3Bsin3B/sinB=sinA/sinB=a/b.