[tex] \frac{1}{1 + 2 + 3 + 4 + ... + 10} = \\ \\ \frac{1}{ \frac{n(n + 1)}{2} } = \\ \\ \frac{1}{ \frac{10(10 + 1)}{2} } = \\ \\ \frac{1}{ \frac{10 \times 11}{2} } = \\ \\ \frac{1}{ \frac{110}{2} } = \\ \\ \frac{1}{55} [/tex]
[tex]\frac{1}{1+2+3+4+...+10}[/tex]
=[tex]\frac{1}{10*(10+1)/2} \\[/tex]
=[tex]\frac{1}{10*11/2}[/tex]
=[tex]\frac{1}{5*11}[/tex]
[tex]\frac{1}{55}[/tex]
