Question

21|(1+2+3+..+20)
este sau nu divizibil?
rezolvarea am facut-o dar nu stiu dacă e divizibil!!! dau coroana​

Answer 1

$\displaystyle\it\\\sum_{n=1}^mn=1+2+3+...+m=\frac{m(m+1)}{2}.\\----------------------\\21|(1+2+3+...+20) \Leftrightarrow 21|\frac{20\cdot21}{2}=10\cdot21 \implies\\evident,~21|21\cdot10.$