有正弦定理可知,c/sinC=b/sinB,c=b*sinC/sinB
(c^2+b^2)/(c^2-b^2)=(b^2*sin^2C/sin^2B+b^2)/(b^2*sin^2C/sin^2B-b^2)
=b^2*(sin^2C/sin^2B+1)/[b^2*(sin^2C/sin^2B-1)]
=(sin^2C/sin^2B+1)/(sin^2C/sin^2B-1)
=(sin^2c+sin^2b)/(sin^2c-sin^2b)
根据正弦定理求证(c^2+b^2)/(c^2-b^2)=(sin^2c+sin^2b)/(sin^2c-sin^2b)
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