given that

=a²-(b-c)²

find    tanA =?

we get=4(s-b)(s-c).

now apply tan(A/2)= /s(s-a)

on sub. we get tan(A/2)=4{tan(A/2)}^2

hence tan(A/2)=0.25

so tan(A)=8/15

ps rate me if im correct