A+B+C=180
2A=360-(2B+2C)
cos2A=cos(2B+2C)=2[cos(B+C)]^2-1
因为cos2X=1-2(sinX)^2
所以(sinX)^2=(1-cos2X)/2
所以sin^2[(B+C)/2]=[1-cos(B+C)]/2
所以8sin^2[(B+C)/2]-2cos2A=7
4-4cos(B+C)-4[cos(B+C)]^2+2=7
4[cos(B+C)]^2+4cos(B+C)+1=0
[2cos(B+C)+1]^2=0
cos(B+C)=-1/2
所以B+C=120
A=60度
由余弦定理
a^2=b^2+c^2-2bccosA
3=b^2+b^2-bc
b^2+c^2=bc+3
b+c=3
b^2+c^2+2bc=9
bc+3+2bc=9
bc=2
b+c=3
所以b=2,c=1或b=1,c=2