ACTUALLY,its a formula,which goes this way: tan(B-C/2) = b-c/b+c hope,u cn solve it now.
( derirvation of the formula:
b-c/b+c = sinB-sinC/sinB+sinC ,
= (2 sin B-C/2 *cos B+C/2) / ( 2 sin B+C/2*cos B-C/2)
= (tan B-C/2) / (tan B+C/2)
now,putting B+C=∩ - A, U GET THE FORMULA