Question

Tính

A= 1/ 3.4 + 1/ 4.5 + 1/5.6 + 1/6.7 + 1/7.8

B= 1 /10 + 1/ 15 + 1 / 21 + ... + 1/120

Answer 1

\(A=\dfrac{1}{3.4}+\dfrac{1}{4.5}+\dfrac{1}{5.6}+\dfrac{1}{6.7}+\dfrac{1}{7.8}\)

\(A=\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{7}-\dfrac{1}{8}\)

\(A=\dfrac{1}{3}-\dfrac{1}{8}\)

\(A=\dfrac{8}{24}-\dfrac{3}{24}\)

\(A=\dfrac{5}{24}\)

Answer 2

A=1/3.4+1/4.5+1/5.6+1/6.7+1/7.8

=\(\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{6}-\dfrac{1}{6}+\dfrac{1}{7}-\dfrac{1}{8}\)

=\(\dfrac{1}{3}-\dfrac{1}{8}\)=\(\dfrac{8-3}{24}\)=\(\dfrac{5}{24}\)

Answer 3

A=1/3.4+1/4.5+1/5.6+1/6.7+1/7.8

A=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8

A=1/3-(1/4-1/4+1/5-1/5+1/6-1/6+1/7-1/7)-1/8{mk ko bít viết phân số thông cảm nha}

A=1/3-0-1/8

A=1/3-1/8

A=5/24

Answer 4

B=1/10+1/15+1/21+...+1/120

B=2/20+2/30+2/42+...+2/240=2.(1/20+1/30+1/42+...+1/240)

B=2.(1/4.5+1/5.6+1/6.7+...+1/15.16)

B=2.(1/4-1/5+1/5-1/6+1/6-1/7+...+1/15-1/16)

B=2.(1/4-1/16)

B=2.3/16

B=3/8

( học tốt nha)