l1：2x-y+a=0 l2:2x-y-1/2=0
l1 l2距离为d d^2=(a+1/2)^2/(2^2+1^2)=(a+1/2)^2/5=(7/10)^2*5
a+1/2=7/2,-7/2
a=3,-4
因为a>0
所以a=3