a22=a1+21d<0
100+21d>0
d>-100/21
a21=a1+20d>0
100+20d>=0
d>-5
-5<=d<=-100/21
ответ d=-5
22=a1+21d<0100+21d>0d>-100/21a21=a1+20d>0100+20d>=0d>-5-5<=d<=-100/21ответ d=-5
a22=a1+21d<0
100+21d>0
d>-100/21
a21=a1+20d>0
100+20d>=0
d>-5
-5<=d<=-100/21
ответ d=-5
22=a1+21d<0
100+21d>0
d>-100/21
a21=a1+20d>0
100+20d>=0
d>-5
-5<=d<=-100/21
ответ d=-5