Question

3+7+11+15+19+...
Sn=210
n=?

Answer 1

$a_1=3;\ a_2=7\ =\ \textgreater \ d=7-3=4\\ a_n=a_1+d(n-1)= 3+4(n-1)=4n-1\\ s_n=\dfrac{a_1+a_n}{2}*n=\dfrac{3+4n-1}{2}*n=(1+2n)n\\ S_n=210\ =\ \textgreater \ (1+2n)n=210\\ 2n^2+n-210=0,\ n \in N\\ D= 1681;\ =\ \textgreater \ n=10$
Ответ: 10