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1 +3+5+...+79 Suma gauss pls

Răspuns :

1 + 3 + 5 + 7 +...+ 77 + 79 =

= 2×1-1 + 2×2-1 + 2×3-1 + 2×4-1 +...+2×39-1 + 2×40 - 1 =

= 2×(1+2+3+4+...+39+40) - (1+1+1+...+1) de 49 ori =

= 2 × 40 × (40 + 1) : 2 - 1×40 =

= 2 × 40 × 41 : 2 - 40 =

= 40 × 41 - 40 =

= 40 × ( 41 - 1) =

= 40^2 =

= 1600




S=1+3+5+........+79
S=79+77+75+...+1
adunând cele două sume
2S=80+80+80+.......+80 de 40 de ori
fiind suma consecutivă de cifre impare
2S=80×40
S=80×40:2
S=40×40
S=40^2=1600