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Care este suma nr 23+27+30+...+ 303?

Răspuns :

23=4×5+3

27=4×6+3

30=4×7+3

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303=4×75+3

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n=4×x+3(x=de cate ori se include 4 in n)

nr termeni=75-4-1=70 termeni

S=70×3+4(5+6+7+...+75)

5+6+7+...+75=75×76/2-1-2-3-4-0

=2850-10

=2840

S=210+4×2840

S=11570