Question

Вычислите: 25+28+31+...+241+244+247

Answer 1

$\displaystyle a_1=25;a_n=247;d=3\\a_n=a_1+d(n-1)=25+3(n-1)=247\\n=75\\S_n=\frac{a_1+a_n}{2}*n=\frac{25+247}{2}*75=10200$