First of all $${{70}^{2}}=4900$$, So the number is between $$70$$ and $$80$$.  According to the last digit, the number should be $$72 $$ or $$78 $$.  $$\because {{72}^{2}}=5184$$，$${{78}^{2}}=6084$$．  $$\therefore a+b=1+8=9$$． 
If a four-digit number $$\overline{5ab4}$$ is a perfect square number, then $$a+b=$$． 
* $$8$$ 
* $$9$$ 
* $$10$$ 
* $$11$$ 