cos²(A/2)+cos2A+1/2=[(1+cosA)/2]^2+2(cosA)^2-1+1/2=[(1+4/5)/2]^2+2*(4/5)^2-1+1/2=159/100∵0∴sinA=√[1-(cosA)^2]=√[1-(4/5)^2]=3/5S△ABC=1/2bcsinA3=1/2*2*c*3/5c=5a=√(b^2+c^2-2bccosA)=√(2^2+5^2-2*2*5*4/5)=√13
