Podaj wzór ogólny ciągu arytmetycznego.Oblicz dwudziesty wyraz tego ciągu.
a)2,4,6,8,10.... b)4,4.5,5,5.5,6... c)3,2,1,0,-1,-2,...
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a)
a1 = 2
r =2
an = 2 + (n-1)*2 = 2 + 2n -2 = 2n
a20 = 20*2=40
b)
a1= 4
r = 0.5
an = 4 + (n-1)*0.5 = 4 + 0.5n - 0.5 = 0.5n +3.5
a20 = 0.5*20 +3.5 = 13.5
c)
a1 = 3
r = -1
an = 3 + (n-1)*(-1) = 3 - n + 1 = -n +4
a20 = -20 +4 = -16