Wyznacz ciąg arytmetyczny wiedząc, że S4=12 i S6=42
(a+a+3r)/2*4=12/:2
(a+a+5r)/2*6=42/:3
2a+3r=6/*(-1)
2a+5r=14
-2a-3r=-6
---------------- +
2r=8
r=4
2a+3*4=6
2a=6-12/:2
a=-3
an=-3+4(n-1)
an=4n-7
Odp. a1=-3, r=4
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(a+a+3r)/2*4=12/:2
(a+a+5r)/2*6=42/:3
2a+3r=6/*(-1)
2a+5r=14
-2a-3r=-6
2a+5r=14
---------------- +
2r=8
r=4
2a+3*4=6
2a=6-12/:2
a=-3
an=-3+4(n-1)
an=4n-7
Odp. a1=-3, r=4