1) progresie aritmetica cu ratia r=4 si (4n-3 -1):4 +1 = n-1+1 = n termeni
Suma termenilor S=n(1+4n-3) : 2 = n(4n-2) : 2 = 2n(2n-1) : 2 = n(2n-1)
2) = (1-1/3+1/3-1/5+...+ 1/(2n-1) - 1/(2n+1) ) x 1/2 = (1 - 1/(2n+1)) : 2 =
= (2n+1-1):2(2n+1) = n/(2n+1)
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