用余弦定理：a^2=b^2+c^2-2bccosA=10c^2-3c^2=7c^2
由正弦定理知：sinA=a/2R,sinB=b/2R,sinC=c/2R.
sinBsinC/(sinA)^2=bc/a^2=3c^2/(7c^2)=3/7.