[Ne[S]zA]

ข้อ1)ครับ
จากโจทย์
$$\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+...+\frac{1}{132}+\frac{1}{156}$$
$$=\frac{1}{6\times7}+\frac{1}{7\times8}+\frac{1}{8\times9}+...+\frac{1}{11\times12}+\frac{1}{12\times13}$$
$$=(\frac{1}{6}-\frac{1}{7})+(\frac{1}{7}-\frac{1}{8})+...+(\frac{1}{10}-\frac{1}{11})+(\frac{1}{11}-\frac{1}{12})+(\frac{1}{12}-\frac{1}{13})$$
$$=\frac{1}{6}-\frac{1}{13}=\frac{7}{78}$$