Question Number 66036 by aliesam last updated on 08/Aug/19
Answered by mr W last updated on 08/Aug/19
$$=\mathrm{2}\left(\mathrm{1}+\mathrm{2}+\mathrm{3}+...+\frac{{n}}{\mathrm{2}}\right) \\ $$$$=\mathrm{2}×\frac{\left(\mathrm{1}+\frac{{n}}{\mathrm{2}}\right)\frac{{n}}{\mathrm{2}}}{\mathrm{2}} \\ $$$$=\frac{{n}\left({n}+\mathrm{2}\right)}{\mathrm{4}} \\ $$