Question

Gasiti si scoateti un factor comun, cand calculati urmatoarele sume: a) 21+42+63+84+....+2100= b)4+8+12+16+...400= c) 30+60+90+120+...+900= d) 15+30+45+60+....+600=

Answer 1

a ) 21 x  ( 1 +2+3+......+100)=
21 x 100x 101 :2 
21 x 50 x 101 =
21 x 5050=
106050

b )4 ( 1 +2+3+4+....+ 100) =
4 x 100 x 101 :2 =
4x 50 x101=
4 x 5050 =
20200


c) 30 ( 1+2+3+...+30 ) =
30 x 30 x31 :2 =
900 x 31 :2 =
450 x 31 =
13950