sin²A＝sin²B+sin²C根据正弦定理∴a²=b²+c²∴A=90º∵sinA=2sinBsinC∴2sinBsinC=1∵cos(B-C)-cos(B+C)=cosBcosC+sinBsinC-(cosBcosC-sinBsinC)=2sinBsinC∴cos(B-C)-cos(B+C)=1...