Given z sub 1 is equal to square root of 2 times open paren cosine of open paren 3 pi over 4 close paren plus i sine open paren 3 pi over 4 close paren close paren and z sub 2 is equal to open paren cosine of open paren pi over 2 close paren plus i sine open paren pi over 2 close paren close paren , what is the value of z sub 1 minus z sub 2 ?