To simplify the expression (0,3 + 4/5)×1/2-3/4, we can follow the order of operations (PEMDAS/BODMAS):

First, we need to simplify the addition inside the parentheses:
0,3 + 4/5 = 3/10 + 4/5

To add these fractions, we need a common denominator, which is 10:
3/10 + 4/5 = (3/10)×(2/2) + (4/5)×(2/2)
= 6/20 + 8/20
= 14/20

Next, we multiply the result by 1/2:
(14/20)×(1/2) = (14×1)/(20×2)
= 14/40

Finally, we subtract 3/4:
14/40 - 3/4 = (14/40)×(1/1) - (3/4)×(10/10)
= 14/40 - 30/40
= (14 - 30)/40
= -16/40

Simplifying the fraction -16/40, we can divide both the numerator and denominator by their greatest common divisor, which is 8:
-16/40 = (-16/8)÷(40/8)
= -2/5

Therefore, the simplified expression is -2/5.