0.2(7-2у)=2.3-0.3(у-6)
1.4-0.4у=2.3-0.3у+1.8
1.4-0.4у-2.3+0.3у-1,8=0
-0.1у-2,7=0|*(-1)
0,1у+2,7=0
у=27
A1=13 a2=0 a3=-1
d=?
S(20)=?
an=a1+d(n-1)
S(n)=((a1+an)/2)*n
a2=a1+d(2-1)=a1+d
d=a2-a1=0-13
d=-13
a20=a1+d(20-1)=13+(-13)*19
a20=-234
S(20)=((13+(-234))/2)*20=(13-234)*10
S(20)=-2210
............................
Х=4+5у
2*(4+5у)-3у=1
8+10у-3у=1
7у=-7
у=-1
х=4+5*(-1)=-1