[tex] \frac{1}{3 \times 49} + \frac{1}{7 \times 9} + \frac{1}{ {21}^{2} } [/tex]
[tex] \frac{1}{147} + \frac{1}{63} + \frac{1}{ {21}^{2} } [/tex]
[tex] \frac{10}{441} + \frac{1}{ {21}^{2} } [/tex]
[tex] \frac{10}{441} + \frac{1}{441} [/tex]
[tex] \frac{11}{441} [/tex]