задачка"1:(2•3)+1:(3•4)+....+1:(2013•2014)+1:(2014+2015)

1:(2•3)+1:(3•4)+....+1:(2013•2014)+1:(2014*2015)=1/2-1/3+1/3-1/4+...+1/2014-1/2015=1/2-1/2015=2013/2015