x=an,
4+11+18+...+х=Sn,
a1=4, d=7,
Sn=(2*4+7(n-1)n/2=(7n+1)n/2=228,
7n^2+n-456=0,
D=12769,
n1=-114/14∉N,
n2=8,
n=8;
a8=a1+7d=53,
x=53.
x=an,
4+11+18+...+х=Sn,
a1=4, d=7,
Sn=(2*4+7(n-1)n/2=(7n+1)n/2=228,
7n^2+n-456=0,
D=12769,
n1=-114/14∉N,
n2=8,
n=8;
a8=a1+7d=53,
x=53.