a1=12.5
d=11.2-12.5=-1.3
an=12.5+(n-1)*(-1.3)=12.5-1.3n+1.3=13.8-1.3n
an<0
13.8-1.3n<0
1.3n>13.8
n>13.8/1.3
n>10.6
n=11
a10=13.8-1.3*10=0.8
a11=13.8-1.3*11=-0.5
a1=12.5
d=11.2-12.5=-1.3
an=12.5+(n-1)*(-1.3)=12.5-1.3n+1.3=13.8-1.3n
an<0
13.8-1.3n<0
1.3n>13.8
n>13.8/1.3
n>10.6
n=11
a10=13.8-1.3*10=0.8
a11=13.8-1.3*11=-0.5