The solution of u rr
​
+ r
1
​
u r
​
+ r 2
1
​
u θθ
​
=0,1 θ,u(3,θ)=11sinθ−38sin 2
θ is: u(r,θ)= 2
a 0
​
+b 0
​
lnr
​
+∑ n=1
[infinity]
​
[(a n
​
r n
+b n
​
r −n
)cos(nθ)+(c n
​
r n
+d n
​
r −n
)sin(nθ)] Find the coefficient b 2
​
. a) 3 b) 6 c) 9 d) 2 e) 0