(1)由余弦定理和正弦定理:sinA(cosB+cosC)=sinB+sinC ==>a/R[(a^2+c^2-b^2)/2ac+(a^2+b^2-c^2/2ab)]=b/R+c/R ==>(a^2+c^2-b^2)/2c+(a^2+b^2-c^2/2b)=b+c ==>b(a^2+c^2-b^2)+c(a^2+b^2-c^2)=2bc(b+c) ==>ba^2-bc^2-b^...