area = 1/2 bsinA(c1+c2)

cosA=b^2+c^2-a^2/2bc

so c^2 -2bccosA +b^2-a^2=0

this is quad in c roots are c1 and c2

so c1+c2=2bcosA

so area = 1/2*bsinA*2bcosA

=1/2b^2sin2A