Sn=((2a1+d(n-1))*n)/2
210=((2*3+4(n-1))*n)/2;
420=(6+4n-4)*n;
420=4n^2+2n;
4n^2+2n-420=0;
2n^2+n-210=0;
D=1681=41^2;
x=(-1+41)/4=10.
Otvet: 10.
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Answers & Comments
Sn=((2a1+d(n-1))*n)/2
210=((2*3+4(n-1))*n)/2;
420=(6+4n-4)*n;
420=4n^2+2n;
4n^2+2n-420=0;
2n^2+n-210=0;
D=1681=41^2;
x=(-1+41)/4=10.
Otvet: 10.