Try this:
SELECT A/B AS STD_u, (C1/C2)+(D1/D2) AS STD_e
FROM (
SELECT ths.tsimHastaSay AS A, shd.tigFrekans AS B, shd.Bagil AS C1, (SELECT SUM(Bagil) FROM SMHASTANEDONEMVERILERI_Y) AS C2,
shd.Vaka AS D1, (SELECT SUM(Vaka) FROM SMHASTANEDONEMVERILERI_Y) AS D2
FROM tsimHastaSay AS ths INNER JOIN SMHASTANEDONEMVERILERI_Y AS shd ON ths.HospitalCode= shd.HospitalCode
) AS DT