1+4+7+.....+x=117
a1=1
r=3
(2a1+(n-1)* r)/2 *n=Sn
(2+(n-1)*3 )/2 *n=117 /*2
(3n-1)*n= 234
3n²-n-234=0
Δ =1+2808=2809
√Δ =53
n=9 lub n=-52/6
x=a9=a1+8r
x=1+8*3
x=25
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1+4+7+.....+x=117
a1=1
r=3
(2a1+(n-1)* r)/2 *n=Sn
(2+(n-1)*3 )/2 *n=117 /*2
(3n-1)*n= 234
3n²-n-234=0
Δ =1+2808=2809
√Δ =53
n=9 lub n=-52/6
x=a9=a1+8r
x=1+8*3
x=25