要使 √(x-1/2) 和 √(1/2-x) 有意义,
则有：x-1/2 ≥ 0 ,1/2-x ≥ 0 ,
可得：x-1/2 = 0 ,
解得：x = 1/2 ,
可得：y = √(x-1/2)+√(1/2-x)+3 = 3 ；
先化简再求值：
原式 = 5x+|2y-1|-√(y-4)²
= 5x+|2y-1|-|y-4|
= 5x+(2y-1)-(4-y)
= 5x+3y-5
= 6.5