7
Solution  The forces acting on charge q at A due to charges q at B
and –q at C are F12 along BA and F13 along AC respectively, as shown
in Fig 1 7 By the parallelogram law, the total force F1 on the charge
q at A is given by
F1 = F 
1ˆr  where 
1ˆr  is a unit vector along BC