Question

Buna!Ajutati-ma va rog la acest ex. 1+4+7+...+97=?​

Answer 1

1+4+7+...+97 =

= (3·1-2)+(3·2-2)+(3·3-2)+...+3·(33-2)

= 3·(1+2+3+...+33) - 2·33

= 3·33·34/2 - 2·33

= 3·33·17 - 2·33

= 33·(3·17 - 2)

= 33·49

= 1617