Charges `5.0 xx ^(-7) C and 1.0 xx 10^(-7) C` are
held fixed at the three cornners A, B, C of an equilateral
. charge at C due to the rest two.

Explanation

`F_(AC)=(9xx10^(9)xx5xx10^(-7)xx1xx10^(-7))/((0.5)^(2)`
`=0.18Nt`

`F_(BC) =(9xx10^(9)xx-2.5xx10^(-7)xx1xx10^(-7))/((0.5)^(2))`
`=-.09Nt`
Net force on C is
`vecF_("Net")=vec(F)_(Ac)+vecF_(BC)`

`|vec(F)_(Net)|=sqrt((F_(AC))^(2)+(F_(BC))^(2)+2(F_(AC))(F_(BC))costheta)`
`[theta=120^(@)]`
`=0.15588Nt`