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an=-1+9*(n-1)
an=-1+9n-9
an=9n-10
9n-10>100
9n>110
n>12,(2)
n=13
9n-10<200
9n<210
n<23,(3)
n=23
Jest to suma wyrazow od 13 do 23.
a₁₃=-1+9*12=107
107+116+125+134+143+152+161+170+179+188+199=1672