充分性：因为cosA=-cos(B+C)所以：-cos(B+C)=2sinBsinC-(cosBcosC-sinBsinC)=2sinBsinC-cosBcosC=sinBsinCcosBcosC+sinBsinC=0cos(B-C)=0B-C=π/2+kπ,因为是在三角形中,所以,B-C=π/2；或B-C=-π/2；无论B-C等π/2...