Question

A divizibil cu 5 ,unde a=2+4+6+......78

Answer 1

a=2+4+6+...+78
=>a=2(1+2+3+....+36)
Suma Gauss = $\frac{n(n+1)}{2}$
In aceasta problema n=36 deci :
a=2×$\frac{39*40}{2}$
=>a divizibil cu 5 deoarece  40 este divizibil cu 5

Answer 2

[tex]\it 2+4+6+...+78=2(1+2+3+...+39) =2\cdot\dfrac{39\cdot40}{2} =39\cdot40 \\\;\\ =1560\ \vdots\ 5 [/tex]