👤

Folosind proprietatile adunarii numerelor naturale,calculati mai rapid:

1+2+3+4...+19+20+21


Răspuns :

1+ 2+ 3+ 4+ 5+...+19+20+21=
(1+19)+(2+18)+(3+17)+(4+16)+(5+15)+(6+14)+(7+13)+(8+12)+(9+11)+(10+20)+21=
20+20+20+20+20+20+20+20+20+30+21=
180+ 51=
231

sau

21·22:2=
21·11=
231