 can be written
  can be written   ∠
 ∠  ("m angle theta"),
  ("m angle theta"),
 where:
  m =  
and
   =
 =  
For 3+i:  m =  =
 = 
        =
  =  or approx. 18.435 degrees.
 or approx. 18.435 degrees.
Now, to find the square root, just take the square root of m and divide the angle  by two:
 by two:
 
  sqrt(3+i) =   ∠
 ∠  
   
or approx.   =  ∠
 ∠  
If you must express this as 'a + bi' form (rectangular form):
     a = m * cos( ) = 1.778*cos(9.218) = 1.755
) = 1.778*cos(9.218) = 1.755  
     b = m * sin( ) = 1.778*sin(9.218) = 0.285
) = 1.778*sin(9.218) = 0.285
so   ≈
 ≈   
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EDIT:  the notation I used really comes from Electrical Engineering, but keep in mind it is just shorthand for  a+bi = 