Đáp án:
`\frac{1}{6}`
Giải thích các bước giải:
`\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}+\frac{1}{110}+ \frac{1}{132}`
`= \frac{1}{4×5}+\frac{1}{5×6}+\frac{1}{6×7}+\frac{1}{7×8}+\frac{1}{8×9}+\frac{1}{9×10}+\frac{1}{10×11}+ \frac{1}{11×12}`
`=\frac{1}{4×5}+\frac{1}{5×6}+\frac{1}{6×7}+\frac{1}{7×8}+\frac{1}{8×9}+\frac{1}{9×10}+\frac{1}{10×11}+ \frac{1}{11×12}`
`=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}+\frac{1}{10}-\frac{1}{11}+ \frac{1}{11}-\frac{1}{12}`
`=\frac{1}{4}-\frac{1}{12}`
`=\frac{1}{6}`
