1) "S_n=\\frac{n(2a+(n-1)d)n}{2}. \\;\\;S_{20}=\\frac{(80+19d)*20}{2}=200."
So, "d=\\frac{20-80}{19}=-\\frac{60}{19}."
2) "S=\\frac{a}{1-r}=\\frac{2}{1-\\frac{19}{20}}=40."
3) "S_n=\\frac{a(1-r^n)}{1-r}."
"S_{10}=\\frac{5(1-2^{10})}{1-2}=5115."
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