Oblicz sumę 9+ 13+ 17+ ⋅⋅⋅+ 81 kolejnych wyrazów ciągu arytmetycznego.
9,13,17,......81
r=a2-a1
r=13-9
r=4
a1=9
an=81
Sn=a1+an\2*n
an=a1+(n-1)*r
81=9+(n-1)*4
81-9=72
72=(n-1)*4 /÷4
18=n-1
18+1=n
n=19
S₁₉=9+81\2*19
S₁₉90\2*19
S₁₉=45*19
S₁₉=855
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9,13,17,......81
r=a2-a1
r=13-9
r=4
a1=9
an=81
Sn=a1+an\2*n
an=a1+(n-1)*r
81=9+(n-1)*4
81-9=72
72=(n-1)*4 /÷4
18=n-1
18+1=n
n=19
Sn=a1+an\2*n
S₁₉=9+81\2*19
S₁₉90\2*19
S₁₉=45*19
S₁₉=855