正弦定理sinA/a=sinB/b=sinC/c,则sinB=b*sinA/a,sinC=c*sinA/a,代入2asinA=(2b+c)sinB+(2c+b)sinC，得a^2=b^2+c^2+b*c,由余弦定理a^2=b^2+c^2-2cosA*b*c，cosA=-1/2，
