1/2 + 1/6 + 1/12 + 1/20 +1/30
= 30/60 + 10/60 + 5/60 + 3/60 + 2/60
= (30+10+5+3+2)/60
= 50/60 = 5/6
\(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}\)
\(=1-\frac{1}{2}+\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{6}\)
\(=1-\frac{1}{6}\)
\(=\frac{5}{6}\)
\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}=\frac{1}{1x2}+\frac{1}{2x3}+\frac{1}{3x4}+\frac{1}{4x5}+\frac{1}{5x6}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}\)
\(=1-\frac{1}{6}\)
\(=\frac{5}{6}\)
1/2+1/6+1/12+1/20+1/30
1/1x2+1/2x3+1/3x4+1/4x5+1/5x6
1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6
1-1/6=5/6
1/2=60/120;1/6=10/60;1/12=5/60;1/20=3/60;1/30=2/30.
1/2+1/6+1/12+1/20+1/30
=60/120+10/20+5/60+3/60+2/30
=60+10+5+3+2/60
=80/60
phần rút gọn bạn tự tính nha....
