a(1)=38.6
a(2)=36.7
a(n)<0, n-?
d=a(2)-a(1)
d=36.7-38.6=-1.9
a(n)=a(1)+d(n-1) <0
38.6-1.9(n-1)<0
-1.9n+1.9<-38.6
-1.9n<-40.5
n> 405/19
n>21 6/19
n=22
a(22)=38.6-1.9*21 = -1.3
