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