[tex]\displaystyle\bf\\21+22+23+...+35=?\\\\Calculam~numarul~de~termeni:\\\\n=35-21+1=14+1=15~termeni.\\\\21+22+23+...+35=\frac{n(35+21)}{2}=\\\\=\frac{15(35+21)}{2}=\frac{15\times56}{2}=15\times28=\boxed{\bf420}[/tex]