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a=18-2b
U5+U8=29
(a+4b)+(a+7b)=29
2a+11b=29
2(18-2b)+11b=29
36-4b+11b=29
7b=29-36
7b=-7
b=-1
a=18-2b
a=18-2(-1)
a=18+2
a=20
Un=a+(n-1)b
U31=20+30.(-1)
U31=20-30
U31=-10