Question

B)10+20+30+.......1230
C)7+14+21+......315
P.S:Dau 5 puncte in plus aceluia care imi si explica cum a facut

Answer 1

b) 10+20+30+.......+1230=.......
dam factor comun pe 10
10(1+2+3+...+123)

1+2+3+....+n are formula n (n+1)/2
In cazul nostru n=123
deci 1+2+3+....+123=123*124/2=
123*62=7626
10*7626=76260

c) 7+14+21+......+315= dsam factor comun pe 7
7(1+2+3+...+45)

Folosim aceeasi formula pentru suma=
45*46/2=
45*23=1035
7*1035=7245

Answer 2

1230/10=123
suma =[(10+1230)x123]/2=1240x123=152520/2=76260
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