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4+8+12+.......+36+40

Răspuns :

DariaKitty21
S=n(n+1)/2
S=40×41/2
S=20×41
S=810
[tex]\displaystyle 4+8+12+...+36+40=4(1+2+3+...+9+10)= \\ \\ =4 \times \frac{10(10+1)}{2} =4 \times \frac{10 \times 11}{2} =4 \times \frac{110}{2} =4 \times 55=220[/tex]