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