11)
 Similarly,
  r
r
a
r
2
2
2
1
2
≅
+

cosθ
(2 12)
Using the Binomial theorem and retaining terms upto the first order
in a/r ; we obtain,
1
1 1
2
1 1
1
1 2
r
r
a
r
r
ra
≅
−

≅
+


−
cos
cos
/
θ
θ
[2 13(a)]
1
1 1
2
1 1
2
1 2
r
r
a
r
r
ra
≅
+

≅
−


−
cos
cos
/
θ
θ
[2 13(b)]
 Using Eqs