由正弦定蠢山理:带春中a/sinA=b/sinB=c/sinC=2R,πR^2=2π,所以森简R=根2,sinB=1/(2*根2),cosB=根7/(2*根2),sinA=根3/2,cosA=1/2,cos(A+2B)=cos(A+B)cosB-sin(A+B)sinB=(cosAcosB-sinAsinB)cosB-(sinAcosB+cosAsinB)sinB=(3-根21)/8