dy=0.5dt-0.5ydt
dy+0.5ydt=0.5dt
(y*e^(t/2))'=0.5 e^(t/2)
(y*e^(t/2))'=(e^(t/2))'
y*e^(t/2)=e^(t/2)+c
y= 1+c*e^(-t/2)
t=0 y=0 c=- 1
y= 1-e^(-t/2)t & gt; 0
y=0.8
0.8= 1-e^(-t/2)
e^(-t/2)=0.2
-t/2=-ln5
t=2ln5