A=π/3,|BC|=7
AB.AC=20
|AB||AC|cosA =20
by cosine rule
a^2= b^2+c^2-2bccosA
|BC|^2= |AC|^2+|AB|^2-2|AB|AC|cosA
|BC|^2+4|AB||AC|cosA= |AC|^2+|AB|^2+2|AB||AC|cosA
|AC|^2+|AB|^2+2|AB||AC|cosA=49+4(20)=129
|AB+AC|^2 =|AB|^2+|AC|^2+ 2|AB||AC|cosA
=129
|AB+AC|=√129