The stationary temperature u(r) between two concentric spheres of radii r 1
​
=2 and r 2
​
=8 satisfies u rr
​
+ r
2
​
u r
​
=0 The temperature on each sphere is u(2)=25 and u(8)=43, respectively. Find the value of u(4). a) 35 b) 41 c) 39 d) 37 e) 33