s20=(a1=a20)*20\2
1990=(a1+a20)*10
199=a1+a1+19*d
199=2*(a1+d)+17*d
199=2*6+17*d
199=12+17*d
187=17*d
d=11,
a2=a1+d, значит a1=a2-d
a1=6-11=-5
s20=(a1=a20)*20\2
1990=(a1+a20)*10
199=a1+a1+19*d
199=2*(a1+d)+17*d
199=2*6+17*d
199=12+17*d
187=17*d
d=11,
a2=a1+d, значит a1=a2-d
a1=6-11=-5