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a=1 b=2
un=a+(n-1)b
35=1+(n-1)2
35=1+2n-2
35=2n-1
35+1=2n
2n=36
n=36/2
n=18
sn=1/2×n(a+un)
=1/2×18(1+35)
=9(36)
324