Question

calculati sumele:
2+3+4+ ......+30=?
9+10+11+.....+30=?​

Answer 1

2+3+4+ ...+30 = (1+1)+(2+1)+(3+1)+...+(29+1) =

= 1·29 + 1+2+3+...+29 =

= 29 + (29·30)/2 =

= 29 + 29·15 =

= 29·(1+15) =

= 29·16 =

= 464

9+10+11+.....+30 =

= (8+1)+(8+2)+(8+3)+...+(8+22) =

= 8·22 + 1+2+3+...+22 =

= 8·22 + (22·23)/2 =

= 176 + 11·23 =

= 176 + 253 =

= 429