1、∵cosA=-根号2/4 ,A是△ABC的角∴sinA=√[1-(-√2/4)²]=√14/4由余弦定理a²=b²+c²-2bc×cosA4=b²+2-2×√2×(-√2/4)b4=b²+2+bb²+b-2=0(b+2)(b-1)=0b=1b=-2(舍去）2、cos(2A+...第一问打错了，是让求sinC,请回答下正弦定理a/sinA=c/sinCsinC=c×sinA/a =(√2×√14/4)/2 =√7/4