\(\frac{1}{3}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\)
= \(\frac{1}{3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{6.7}\)
= \(\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{6}-\frac{1}{7}\)
= \(\frac{1}{3}+\frac{1}{3}-\frac{1}{7}\)
= \(\frac{11}{21}\)
Tính nhanh:
A=\(\frac{1}{90}-\frac{1}{72}-\frac{1}{56}-\frac{1}{42}-\frac{1}{30}-\frac{1}{20}-\frac{1}{12}-\frac{1}{6}-\frac{1}{2}\)
B=\(\frac{3}{2}-\frac{5}{6}+\frac{7}{12}-\frac{9}{20}+\frac{11}{30}-\frac{13}{42}+\frac{15}{56}-\frac{17}{72}\)
tính nhanh\(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\)
\(\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}\)Tính nhanh nếu được
giúp mình giải bài này nữa
tính nhanh : \(A=\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}\)
Tính nhanh
\(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56};\)
\(\frac{3}{4.7}+\frac{3}{7.10}+\frac{3}{10.13}+.......+\frac{3}{49.51};\)
\(\frac{3}{1.3}+\frac{3}{3.5}+\frac{3}{5.7}+.......+\frac{3}{49.51};\)
\(\frac{1}{100.101}+\frac{1}{101.102}+\frac{1}{102.103}+......+\frac{1}{2010.2011}\)
tính \(A=-\frac{1}{20}-\frac{1}{30}-\frac{1}{42}-\frac{1}{52}-\frac{1}{72}-\frac{1}{90}-\frac{1}{110}\)(tính nhanh)
Câu 1. Tính nhanh
\(\frac{1}{1\cdot2\cdot3}+\frac{1}{2\cdot3\cdot4}+\frac{1}{3\cdot4\cdot5}+.....+\frac{1}{37\cdot38\cdot39}\)
Câu 2. Tính nhanh tổng A=
\(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+...+\frac{1}{990}\)
Tính nhanh
\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{9.10}\)
\(B=\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}\)
\(C=\frac{1}{15}+\frac{1}{35}+\frac{1}{63}+\frac{1}{99}+\frac{1}{143}\)
Ai nhanh tôi lì xì cho :
A = \(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}\)