20. The net electric potential at point P is the sum of those due to the six charges:
V
P
=
6
i=1
V
P i
=
6
i=1
q
i
4πε
0
r
i
=
1
4πε
0
5.0q
d
2
+ (d/2)
2
+
−2.0q
d/2
+
−3.0q
d
2
+ (d/2)
2
+
3.0q
d
2
+ (d/2)
2
+
−2.0q
d/2
+
−5.0q
d
2
+ (d/2)
2
=
−0.94q
4πε
0
d
.