= 1/2 + (1/2 -1/3) + (1/3 - 1/4) + (1/4 - 1/5) + ( 1/5 - 1/6) + (1/6 - 1/7)
= 1/2 + 1/2 - 1/7
= 6/7
1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42
= \(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}\)
= \(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)
= \(1-\frac{1}{7}\)
= \(\frac{6}{7}\)
Lưu ý : dấu " \(.\)" là dấu nhân nha
\(\frac{1}{2}\)+ \(\frac{1}{6}\)+ \(\frac{1}{12}\)+ \(\frac{1}{20}\)+ \(\frac{1}{30}\)+ \(\frac{1}{42}\)
= \(\frac{1}{2}\)+ ( \(\frac{1}{2}\)- \(\frac{1}{3}\)) + (\(\frac{1}{3}\)- \(\frac{1}{4}\)) + ( \(\frac{1}{4}\)- \(\frac{1}{5}\)) + ( \(\frac{1}{5}\)- \(\frac{1}{6}\)) + (\(\frac{1}{6}\)- \(\frac{1}{7}\))
= \(\frac{1}{2}\)+ \(\frac{1}{2}\)- \(\frac{1}{7}\)
= \(\frac{6}{7}\)
tk nhé
